Request a global account rename

As a user who is going to be forcibly renamed
I would like the opportunity to choose a new name that I like
So that I'm not stuck with OLDNAME~wiki as my new name.

Provides a new Special:GlobalRenameRequest page that can be used to
submit a rename request to the global rename queue. Also adds a check
for pending rename requests to the existing AbortNewAccount hook.

The feature is guarded using the $wgCentralAuthEnableGlobalRenameRequest
global flag.

Bug: 68886
Change-Id: I5a657934453f99aa189d795988564b8f1cfa9633
Merge "Request a global account rename"