private-messages: Update frontend strings to use "direct message" or "DM".

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.

web/src/compose_validate.js

@@ -501,7 +501,7 @@ function validate_private_message() {
     ) {
         // Unless we're composing to a bot
         compose_banner.show_error_message(
-            $t({defaultMessage: "Private messages are disabled in this organization."}),
+            $t({defaultMessage: "Direct messages are disabled in this organization."}),
             compose_banner.CLASSNAMES.private_messages_disabled,
             $("#private_message_recipient"),
         );