Various fixes to overlay header code

Mainly CSS, fixes broken language search input, also:
* replaces non-existent message with the correct one
* uses mw.msg() instead of M.message()
* removes options.locked in createOverlay() (unused)

Change-Id: I3ffb4643f91b1d4ee3c6d57014a8dc7736a97a2f
Merge "Various fixes to overlay header code"