Extract i18n strings from .tpl files as well

These are used e.g. for email reminders and are not .html so they need to be
specified explicitly.

Fixes #179

docs/i18n.rst

@@ -11,7 +11,7 @@ wger uses django's translation infrastructure, but there are a couple of things
 that need to be considered. First, update your po files with the usual django
 command (run this in the wger subfolder, not the root one)::
 
-    django-admin makemessages --locale en
+    django-admin makemessages --locale en --extension html,tpl
 
 Extract some translatable strings from the database such as exercise categories
 and muscle names::