Editor switcher

Authored by EBernhardson.


Editor switcher

Create buttons to switch between wikitext and visual editors. Remembers the
selected editor and continues to use that on future requests. This removes
any user visible references to previewing, and instead has a link in the
wikitext editor help that switches to VE for previews. A followup will remove
the various preview related code that is now unused.

Bug: T90763
Change-Id: Ibed8693bfd825819aa53fec61385303b29cecfb1