New-style stream/topic recipient bars

[gnprice]

This is a sub-issue, for the Beta 1 milestone, of:

• Convert message list to new, 2023+ design #157

For the first beta, we'll want this change in particular:

Stream/topic recipient bars have the stream color the whole way across, are taller, have the stream name and topic in bold, and other changes.

because it's among the most conspicuous changes in the new-style message list.

Getting the exact variant of the stream color that's used in the design may be complicated. For Beta 1 (and therefore for this issue), we'll keep it simple: it'll be enough for now to lower the opacity of the stream color, or some comparable approximation. We can leave a TODO(#157) comment for that.

If there are other details of the design that also turn out to be complex to implement, then we'll similarly want to leave those out with a TODO(#157) comment.