Adds support in the web app for `dm` operator. This will deprecate
the `pm-with` operator, but existing links/URLs are still supported
for backwards-compatilibity.
This commit updates the web app default behaviors to default to
the new narrow/URLs `dm/...` and `/#narrow/dm/...` when navigating
and searching in the app.
There is some general clean up of references to private messages
or PMs to be either direct messages or DMs in these changes.
The general API changelog and documentation updates will be done
in a final commit in the series of commits that adds support for
the various new direct message narrows.
Updates frontend user-facing strings with "private message" or "PM" to
use "direct message" or "DM" respectively instead.
Note that this updates translated strings as well as a few that
are not translated like search suggestions.
Updates `tools/lib/capitalization.py` for some specific strings
that are impacted by these changes, and removes "PM" and "PMs"
from checked strings.
Lauryn Menard
Anders Kaseorg