Implement OAuth user checks

Implement the OAuth cross-wiki identity hooks, so we can ensure the
user who authorizes an OAuth Client on one wiki is the same user who
is using it on potentially a different wiki.

OAuth change Ic274ff3d65ecfa9d602fb92b8998276ce722f5b1 also adds the
hook OAuthGetUserNamesFromCentralIds.

Change-Id: I71632de70d7364c52816749b26c9dfcb3cfd67cc
Merge "Implement OAuth user checks"