Requests for comment/Opt-in site registration during installation

Description

 * Related bugs: Provide an opt-in ability to register the user's MediaWiki installation, Create Ping server extension for MediaWiki, Create ping.wmflabs.org, Expose proposed ping info in the API, Ask for wiki logo during installation, Ask for oneline wiki tagline during installation, Ask for wiki description during installation

Rationale
Currently there is no method to collect statistics on wiki installations built into MediaWiki. This means that a lot of wikis get installed and, unless they are discovered somehow, tools that track wiki installations (like wikistats or WikiApiary) cannot find them. This is a method that would enable us to better estimate the number of users of MediaWiki.

Privacy
In an effort to preserve privacy, this should be an opt-in feature. That is, the end user must purposefully checks a box saying “ping wikimedia” (and it is made clear what information is being sent in the ping) before the ping is executed.