Fixed #21388 -- Corrected language code for Frisian
Fixed #21421 -- Added level_tag attribute on messages.

Exposing the level name (e.g. "info") makes it possible to prepend
something to the class name. For example, Twitter Bootstrap has
an alert-info class. This class can now be added to the message
using `class="alert-{{ message.level_tag }}".
Because the level_tag was on the end of the `tags` property, it
could not be used in this fashion when extra_tags were given.
Fixed #21423 -- Fixed typo in widgets.py.
Merge pull request #1907 from Bouke/tickets/21388

Fixed #21388 -- Corrected language code for Frisian
Flake8 fixes -- including not runnign flake8 over a backported file