Fixes#2220
This drops the ability to shift+click on “Back” to get back to a pinned
column, but that was inconsistent, broken, and undocumented.
This also brings us slightly closer to upstream.
Using an emoji map was completely unnecessary in
the first place, because the reaction list from
the API response includes URLs for every custom
emoji anyway. The reaction list now also contains
a boolean field indicating whether it is an
external custom emoji, which is required because
people should only be able to react with Unicode
emojis and local custom ones, not with custom
emojis from other servers.
This also adds the comment in action_bar.js to
status_action_bar.js, clarifying that a future
version could improve this code by modifying
EmojiPickerDropdown.
