stream/topic autocomplete (e.g. #**mobile**)

[chrisbobbe]

After #49, which is likely to inform the architecture here.

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When picking this up, let's plan to ping Vlad on CZO to see if he feels strongly about a design; otherwise we can try just stitching something together from other kinds of autocomplete that are already in Figma.

[ryanwwest]

I'm excited for this feature!

[gnprice]

For anyone picking up this issue: the PR #884 has previous work that might be helpful to borrow from. See Zulip's guide to continuing unfinished work: https://zulip.readthedocs.io/en/latest/contributing/continuing-unfinished-work.html

[alya]

https://github.com/zulip/zulip/issues/36370 has a plan for updating channel ordering in the web app typeahead. We can have refining the ordering be a follow-up in the mobile app, but could maybe pull in some basic ideas, like handling deactivated and unsubscribed channels, if we aren't doing that already.

[sm-sayedi]

Yeah, that would be a good follow-up. Right now in #1902, we do follow the criteria in typeahead_helper.sort_streams for sorting in autocomplete.