Global merge tool

Allows for a list of global users to be merged
into one "final user". This will help after
SUL finalisation where one user may end up with
multiple global accounts.

This follows a similar structure and re-uses
code from GlobalRename.

Introduces Special:GlobalUserMerge for stewards
to use, and re-uses Special:GlobalRenameProgress
for users to use. This special page can be
disabled through $wgCentralAuthEnableUserMerge.

We inject LocalUserMergeJobs into local wikis
which uses SpecialUserMerge to update the
relevant local tables.

Depends on I2bfcecad9 in UserMerge (merged)

Bug: 47918
Change-Id: Ie76c28234b24e66c708ad0dad03251a7e80ea531
Merge "Global merge tool"