activitypub: c2s: support using `source` field to reformat posts like friendica does
This implements an extension to our AP C2S implementation that reformats the content using Pleroma's formatter engine when content
is wrapped in a source
object like so:
{"source": {"content": "***cool post***", "mediaType": "text/markdown"}}
If content
is left as bare HTML it will continue to be left alone as it is now (except for harmful items being stripped from it).