Accept richer text from remote posts

Support abbr, del, pre, blockquote, code, strong, b, em, i, u, ul, ol, li and h1 to h5 tags in remote posts.

Such posts can already be authored in Pleroma and other software, and would currently look something like:

Improperly formatted post (as currently displayed)

This PR makes it so it is displayed like this:

The PR also includes a couple options to visually fallback to a text representation (through client-side CSS):

Block tags “stripped”

Everything “stripped”