Move getPrevious/NextRevision logic out of Title

Move getPrevious/NextRevision logic out of Title

They belong in RevisionStore. This change removes the dependency on
Title for these methods and will assist in porting more code to
LinkTarget.

At the same time, deprecate the Title parameter to
RevisionLookup/RevisionStore getPreviousRevision/getNextRevision, and
add a $flags parameter to match the functionality of the Title versions.
Since code search turned up no callers that passed a Title outside core,
this variant is immediately hard-deprecated. The Title methods
themselves are only soft-deprecated.

Change-Id: I76bc6fd6ee1a9f35b5f29fa640824fb5da3bb78e