This converts the CategtoryTree extension to the new (MW1.35) hook system.
The patch was written live during a hackathon workshop.
A recording of the workshop is available at
<https://www.youtube.com/watch?v=ZOj44Rbh0tM&t>.
Re-applying after fixing: premature access to $wgRequest
Restores change: Ie52c393af378a980a2dac4ae7076fd6c016a8e0e
Reverts revert: Ieee300c7b35b7069bd7e781610c915f2ecae1bf1
NOTE: the "mode" parameter for category pages seems to be broken,
as the mode is not proeprly propoagated to dynamic requests.
This was already the case before this change, and this change
does not try to fix that issue. I was tempted to just remove
the feature entirely, but T137812 inidcates that it may be useful to
some. I will comment there to see if this should be fiexed or removed.
Bug: T282110
Bug: T271011
Change-Id: Ic3ee05e9dc382f4ada5cfeb9ff5d9a69249cc60d
