Add @mentions in chat-box

[shahyar]

Using onKeyUp, if key is Tab, and @ exists before the caret, use that entire string to look up a user by their name. ie. @foo bar will find user Foo Bar, @foo will find both Foo Bar and fooman.

Also removes initializing deleted users in SlackAccount.ready.

This depends on https://github.com/yue/yue/pull/44 to actually work, but this code should not break anything without it merged (since the selected index will be -1 to -1).

Known issues:

• View.onKeyDown is only triggering for modifier keys (MacOS only?). This requires a hack that we have to delete the inserted tab instead of preventing preventing it entirely.
• Menu.popup() does not allow you to specify a location, so the users list appears at the mouse cursor, instead of the text caret.
• There is no way to show the "pretty" user name at the moment. In a future version, we could potentially show the username as usual, while keeping track of the its string position, then render a highlight on top -- and finally re-process the message on send to inject the stored <@id>.
• This does not yet implement UAX44-LM3, so if you want to look up a user named ère, you'll have to type @èr not @er.

[shahyar]

Here's a brief video showing how this works, and its caveats: https://imgur.com/a/KW9SQ2k

[tuananh]

@shahyar i love PR with video :D

[zcbenz]

It seems that #65 already includes this change, so I'm closing this one.