Simplify the buffers by making everything work on a TalkatuBuffer

The only things the content type specific buffers were doing now as defining
which formatting actions would be enabled. So we now created an flags and a
property on TalkatuActionGroup to fix that. By doing so, we are able to get
rid of TalkatuHTMLBuffer, TalkatuMarkdownBuffer, and TalkatuWholeBuffer.

Testing Done:
Ran the demo, tested all the buffers as well as the insert html and mark down actions.

Reviewed at https://reviews.imfreedom.org/r/2224/