This fixes missing translation tags in our missed-message emails,
which is surely the most user-facing part of the production that
wasn't internationalized.
Fixes: #14398
This is a dramatic redesign of the look and feel of our missed-message
emails, designed to decrease the feeling of clutter and just provide
the content users care about in a clear, visible fashion.
Apparently, when we renamed these files to no longer have a .txt
extension, we accidentally removed them from the set of strings for
translation, because `manage.py makemessages` by default only
processes .txt and .html files under the templates/ directory.
Fix this by adding a .txt extension.
vaibhavrajsingh2001
Vishnu Ks
Soumya Himanish Mohapatra
Tim Abbott