This addresses another usability problem from #470.
I decided it was too confusing to have menu options for both stream and subject
narrowing, so I went with the latter.
(imported from commit 948fca954a265029ceb27c72ae178015d513785d)
The message timestamp is now always clickable, and the popover contains the
full long-form date and time. This addresses one problem from usability
testing (see #470).
(imported from commit ad502dff128ad1c934fc0d3faaf5e2931c91c37e)