Final diff for MVP typeahead for web.
We lack some nice UX from design, but it can be done in future diffs after
we implement typeahead on native.
Nice to haves (that we don't have yet):
- keyboard support (choosing with arrows, confirming with Enter, cancelling with Esc), for now you gotta click username with your mouse
- @ing people that are not in a channel thread would create a modal asking if you want to invaite them
See a video for how it feels and works:
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