This is a follow-up to merging the compose and reactions emoji
pickers. The logic for what happens when the user picks an emoji via
the hotkeys (i.e. hits `enter`) was still attempting to add a reaction
to the currently selected message unconditionally.
This commit adds a check in the two `enter` key code paths, and does
the correct thing in each case.
Fixes#4736.
This removes the old compose emoji picker in its entirety, changing
the few callbacks needed to launch the reactions-style emoji picker
instead and hook it up properly.
Callbacks for reactions and composing messages are distinguished by
selecting for, respectively, the .reaction and .composition classes.
Fixes#4122.
Tejas Kasetty
Jack Zhang
Tim Abbott