Warn users when they are typing wikitext

Without making the code much more complex (and possibly create
performance issues) the warning will fire on pages which already
contain escaped wikitext (when that text is edited). I think this
should be a small enough minority that it won't be an major annoyance.

Bug: 49820
Change-Id: I0f67ec04b890f4add9247be6126bdc086b6ae72f
Merge "Warn users when they are typing wikitext"