Support manual thumbnail option (thumb=...) on images.

Use a data-mw attribute (data.mw.thumb=Thumb.png) to record the name
of the manual thumb, since backing out the thumb title from a scaled
img src is problematic.

Change-Id: I91d170aa4f6d1fd844d24ccb85b886eb079e3e2a
Merge "Support manual thumbnail option (thumb=...) on images."