Requests for comment/Reimplement info action

This is a request for comment regarding reimplementing the info action.

Background
The "info" action has existed in MediaWiki since 4242. Since 4247, the action has been disabled by default behind the  global variable due to performance issues.

Reimplementation
The "info" action should provide human-readable metadata about a particular page title (e.g., "Main Page" or "Barack Obama"). Some of this data is a view into the page_props table; other data is simply interesting to editors and readers.

The data contained in this info dashboard will likely need to be split between expensive and inexpensive queries. For larger wikis, $wgMiserMode can be relied upon to block the expensive queries.

Inexpensive properties

 * DEFAULTSORT value
 * DISPLAYTITLE value
 * (NO)INDEX status
 * LiquidThreads status
 * page protection status (related to a separate bug)
 * first edit timestamp
 * page creator
 * last edit timestamp
 * most recent editor
 * templates transcluded on this page
 * hidden categories on this page

Expensive properties

 * number of watchers
 * number of total edits
 * number of unique contributors (anonymous and logged in)
 * number of views (link to outside service?)

User interface
Currently the "info" action is not exposed in the user interface, regardless of the value of $wgAllowPageInfo (16362). A tab or a link in the sidebar in the "Toolbox" section should provide easy access to the "info" action.

Plan

 * File tracking bug about reimplementing the info action
 * 38450
 * File bug about killing $wgAllowPageInfo and enabling the "info" action by default
 * 38451
 * File (many) bugs about adding specific data to the action, as detailed above