Add support for sending room-local emoji (#209)

Add support for sending room-local emoji (#209)

* Add support for sending room-local emoji

Does not add support for sending a room's emoji outside of that room, but enables users to
send an emoji if the packs in a room support it.  Does not include room emoji in the
picker YET.

* Amend PR #209: Don't freak out if the `pack` tag is missing

* Amending PR:  Refactor emojifier, use better method for retrieving packs

* Amending PR:  Improve resiliance to bad data in emoji state events

* Amend PR: Remove redundant code, fix crash on edit