Removed existing empty narrow divs from app/home.html and created
a new javascript module to dynamically load empty narrow messages
using handlebar template.
Fixes#18797
The default label for empty narrows depends on whether it's a
stream/topic narrow or a PMs narrow.
We leave some default text in compose.hbs for reply label
because it take some time for the js to display the
correct label.
This is a mostly verbatim extraction.
I re-phrased one line of code to work around a lint
false alarm. (Look for `preamble` in the diff.)
There are about 8 lines missing coverage here, so
the new module might be a good candidate to get
100% line coverage on.
Before this change, you would need to remove 74
edges from our dependency graph to make it
acyclic. Now it's 72.