Multiple mentions autocompletion UX

[smichel17]

Polari, the GNOME IRC client, has the best mention autocompletion UX, that I'd love for Riot to emulate.

• In both Polari and Riot, when you mention someone at the start of the message and tab-complete it, their name (pilled in Riot) is followed by a colon and a space.
• In Riot, when you tab-complete a second person's name immediately afterward, it simply pills it, as if you were mentioning them in the middle of a sentence.
• In Polari, it will add a colon after the second person's name, and change the first colon to a comma.

Here is a screencast of the polari behavior (it's annoyingly blurry unless viewed at native resolution, sorry about that):

And here is polari's autocompletion code (GPLv2)

I know it seems like a minor feature, but I use it so much that it's seriously important to the UX for me; it's pretty much the sole feature keeping me split between Riot and Polari, rather than using Riot exclusively. Please, go try it yourself; the screencast doesn't really do it justice. It's the kind of thing where, after you've used it, it's so glaringly obvious that this is the Right Way To Do It™ that you wonder how anybody did it any other way before. Like microwaves that combine the start and minute plus button.

[turt2live]

Duplicate of #4883

[smichel17]

Ack, I did search first, but didn't find that one :(