Make streams expandable/collapsible in Streams panel

In preparation for the mobile app redesign, we should implement the following functionality for the Streams panel:

1. Each stream should be expandable/collapsible within the panel, similarly to #5423. Clicking on the stream name should expand/collapse it.
2. When a stream is expanded, we should use the logic used in the left sidebar of the webapp to decide how many topics to show.
3. An expanded stream should have a "more topics" link at the bottom. Clicking on this link should take the user to the full-screen topics list for that stream (currently accessed via the three bulleted lines icon in the stream header bar).

Note that unlike the left sidebar of the webapp, it should be possible to have several streams in the expanded state.

Design direction (visual redesign will be a separate follow-up issue):