Only create editor switcher from wikitext editor if VE is usable

Prevents creation of the editor switcher from the wikitext editor if:
* VE is diabled for current namespace
* Page is output of Page Translation feature
* Page doesn't use wikitext content model

Also improving comment about save warning.

Bug: 64681
Change-Id: Ib39e76d0e59328c18b1937157d0c9d11efa16eda
Merge "Only create editor switcher from wikitext editor if VE is usable"