compose: Error banner for deactivated DM recipient has lost its top margin

[chrisbobbe]

In 3adb395 (fyi @PIG208), we regressed on the layout of the error banner shown when a DM recipient is deactivated:

Before	After

The top margin disappeared, and it looks buggy.

I plan to fix this in a PR that's mostly focused on proposing some prep refactors to make a cleaner starting point for #720. (If those refactors seem good, Zixuan can rebase #924 past them.)

[PIG208]

Thanks to sending this issue! eb9fccd (from #924) intersects with this by using the same _ErrorBanner class for the error banner here and the one that appears above the compose box.

I noticed that I didn't post a screenshot for this. Added it to #924. Also starting a discussion on the design of this.