Improve stream topic view

[alya]

At present, it's possible to view the list of topics in a stream by clicking the three-dot/line icon the stream header bar. We would like to make this view more prominent, likely as a default stream view. As a pre-requisite, we should make some improvements to this view:

1. Add a stream header bar. This should be the same as the bar we have in the interleaved stream view, but with a different icon replacing the three-dot/line icon. That icon should take the user to the interleaved stream view. I'm not sure what icon to use -- perhaps three lines without dots?

2. Move search to a separate view. The primary use of the list of topics is not search, so we should move topic search to a separate view, which will be accessed via a new magnifying glass icon in the header bar.

3. Make topics list more clickable. At the moment, it's not very obvious that one is intended to click on topics; they parse more as a bunch of text. We can try to address this by adding decorators (perhaps >) and increasing line spacing. The latter will also decrease the chance of mis-clicks. (For example, channel lists in Slack and Discord have decorators and much more spacing.)

Related issues: #4529, #4535.

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Update: We will not need a special icon in (1) or decorators in (3); this is the redesign direction: