Preserve subject in replies.

This also adds "re:" to the beginning of the subject if "re: " or "re " isn't already there, to make the "it's like email" comparison that much more real.

changed the description

merged

mentioned in commit 256aa25a

i'd make it configurable tbh

mentioned in commit lanodan/mastofe@8433bd89

mentioned in issue pleroma#223 (closed)