EmojiReactValidator: fix emoji qualification

Supersedes !3684 (merged) with changes to address concerns as to used logic.

Tries fully-qualifying emoji when receiving them, by adding the emoji variation sequence to the received reaction emoji.

This issue arises when other instance software, such as Misskey, tries reacting with emoji that have unqualified or minimally qualified variants, like a red heart. Pleroma only accepts fully qualified emoji in emoji reactions, and refused those emoji. Now, Pleroma will attempt to properly qualify them first, and reject them if checks still fail.

This commit contains changes to tests proposed by lanodan.