Extension:CentralNotice/Notes/Banner controller refactoring

Here's a place to keep some rough notes about reorganizing CentralNotice banner controller code. See also Campaign-associated mixins and banner history. Work should go on the  branch.

Goals

 * Improve modularity and separation of concerns in banner controller code.
 * Adapt the banner controller API as needed for new campaign-associated mixins.
 * Improve site performance when banners are shown and when they're not.

Requirements

 * Separate concerns in a way that makes sense for selective loading of components.
 * Follow typical and best-practice MW patterns.
 * Remove cruft.
 * Prepare to move out code that has uses beyond CentralNotice (GeoIP cookie stuff, maybe mobile device detection).

Proposal
Following are some possible objects and their responsibilities. Format is: RLModule/object.

ext.centralNotice.bannerController/init Checks  and the presence of a banner URL param, and starts the appropriate processes.

ext.centralNotice.bannerLoader/manager Coordinates other objects and provides an API for outside code, including banners and campaign-associated mixins. (This could be called "controller", but that'd be confusing, since we need to keep bannerController as the module that brings in everything else on the desktop.)

ext.centralNotice.bannerLoader/chooser Selects a campaign and a banner based on.

ext.centralNotice.bannerLoader/bucketer Retrieves, stores, chooses and processes buckets.

ext.centralNotice.kvStore/kvStore Stores and retrieves items for different contexts (campaign, category, global). Should be declared as a dependency by mixin modules, as needed. See also the existing KV store patch.

ext.centralNotice.mobile/deviceChecker Device checking (for mobile skin only).

ext.centralNotice.geoIP/geoIP Sets window.Geo. May be moved to a different extension or to MW core. (See the related Phabricator task.)

ext.centralNotice.testingBannerLoader/testingBannerLoader Small module for loading banners for testing purposes. Loads dynamically via mw.loader.load when a banner parameter is included in the URL.

We'll use dynamic RL dependencies of  to select the RL modules needed.

Requirements

 * Remain compatible with existing banners. Certain methods, properties and hooks should remain available. We can use JS  to warn when deprecate properties are accessed. The following should stick around:
 * Here are things that we don't need to keep publicly accessible outside CentralNotice:
 * (already deprecated)
 * (already deprecated)
 * New stuff the API will offer for campaign-associated mixins:
 * Retrieve bucket.
 * Set bucket.
 * A flag that can be set by a mixin to signal that banners in a campaign are guaranteed to display to the user if loaded. This will disable Special:RecordImpression for such campaigns and add a parameter to the call to Special:BannerLoader to confirm that impressions may be counted using logs to that endpoint.
 * A flag that can be set by a mixin to cancel the display of banners from this campaign.
 * Access to the Key-Value store methods.
 * Everything should work smoothly with existing cached HTML.
 * (already deprecated)
 * (already deprecated)
 * New stuff the API will offer for campaign-associated mixins:
 * Retrieve bucket.
 * Set bucket.
 * A flag that can be set by a mixin to signal that banners in a campaign are guaranteed to display to the user if loaded. This will disable Special:RecordImpression for such campaigns and add a parameter to the call to Special:BannerLoader to confirm that impressions may be counted using logs to that endpoint.
 * A flag that can be set by a mixin to cancel the display of banners from this campaign.
 * Access to the Key-Value store methods.
 * Everything should work smoothly with existing cached HTML.
 * Set bucket.
 * A flag that can be set by a mixin to signal that banners in a campaign are guaranteed to display to the user if loaded. This will disable Special:RecordImpression for such campaigns and add a parameter to the call to Special:BannerLoader to confirm that impressions may be counted using logs to that endpoint.
 * A flag that can be set by a mixin to cancel the display of banners from this campaign.
 * Access to the Key-Value store methods.
 * Everything should work smoothly with existing cached HTML.

Proposal
Get the bucket for this campaign. Guaranteed to be available in the  hook.

Will influence which banner is selected when called from the  hook.

Boolean property; default false. Should be set from the  hook.

Boolean property; default false. Should be set from the  hook.

Available key-value storage contexts

Reducing the size of top-loading RL modules

 * Does the fact that RL modules are cached in localStorage somewhat mitigate performance gains from doing this?
 * Why should any CentralNotice code be top-loaded?
 * The sooner the bannerController runs, the less of a page bump there will be when a banner is shown. Also, completely eliminating CentralNotice top-loaded modules would increase the number of round trips across the board.