Rich Text Redo Branch

This reverts the revert, let's continue work on rich text support in this branch.

things to do

• BBCode instead of Markdown (arbitrary HTML is too problematic)
• MRF policy to strip incoming rich text (except links) (MR !345 (merged))
• configurable restriction of accepted content types
• user preference to disable rich text (except links) (MR !354 (merged))
• preview API (to show how the server will render the text exactly)

backend security concerns

• inline images need to go through mediaproxy at the very least (MR !346 (merged))
• regression tests for additional common XSS vectors (onerror attribute and similar) (MR !339 (merged))

pleroma fe

• move rich-text language selection into preferences, use on/off switch
• preview pane

masto fe (stretch goal tbh)

• improve handling of rich-text (MR mastofe!2 (merged))
• authoring support for rich-text
• preview pane

overall

• documentation