Throw a dialog box up to ask whether to convert something with formatting to
wikitext, or downconvert it to plain text.
This logically depends on Ie9aaaa59e9dfa138d394051fe491573253df1805.
Bug: T190079
Change-Id: I6afbbe303d1506426109e75c95f6be546ec48536
Depends-On: Ie9aaaa59e9dfa138d394051fe491573253df1805