Refine autocomplete username suggestions

[timkinnane]

On open.rocket.chat or any server with many users, the autocomplete username suggestions after @ are often not very useful, I think for two reasons:

1. The search includes matching on email address - I don't see any reason for this and it just pollutes the list with irrelevant usernames.
2. The search isn't sorted to promote users in the room, so the person you're most likely trying to address often won't be in the list at all.

On the first point, I know it's possible to configure this, but the default configuration should provide the best experience, so I would suggest removing email from default search fields. I don't see why it's even an option, when you're trying to "at" someone, you can't use their email and wouldn't usually know it anyway. Isn't it also a privacy issue, because you could detect someones email address from the suggestions?

This was created from discussion on #10132, but there's many other relevant issues describing a poor experience with search completion. e.g. #10791, #1446, #10004, #874, #2980.