Offer search box in "Channels" page

[chrisbobbe]

A follow-up to #1406, which says:

The [Figma design] shows a "Filter channels" search box at the top. That is a useful feature but will be a separate follow-up issue.

So here is an issue for that.

[chrisbobbe]

A review from an Android user requested this feature today (link for those with access).

[sarafarajnasardi]

@chrisbobbe I would like to work on this issue...

[Sarthak45-del]

@chrisbobbe i am interested on working on this issue pls assign me this issue

[gnprice]

@sarafarajnasardi @Sarthak45-del If you'd like to contribute to Zulip, please take the time to read our contributing guide:
https://zulip.readthedocs.io/en/latest/contributing/contributing.html#finding-an-issue-to-work-on

[gnprice]

Requested by a user in chat last week, along with #1903 for the DMs page:
#mobile > channels and dm filter list @ 💬

[sarafarajnasardi]

Hi @chrisbobbe,
Quick question: Should the filter search box also be added to the All Channels page? I noticed it's in the Figma design there too. Happy to extend this PR to cover that as well if needed.

[gnprice]

Yeah, let's have this issue cover the search box for both of the channels pages.