Various code cleanups

* Created getLocalUserFromCentralId/getCentralIdFromLocalUser functions
* Inject the DB object into the control classes
* Made Special:MWOAuthManageMyGrants use the pretty grant names
* Removed some unused code
* Added various comments
* Other small fixes

Change-Id: I910f5478f2de494026256384115800c3dcc51c2f
Merge "Various code cleanups"