A better way to make all translation strings plain text.

By making _() return an escaped Markup object, we can mark HTML chunks
with HTML(), and have logical .format() semantics without a specialized
and too-narrow helper like format_html.
Convert lots of translation strings to the new way to do plain-text translation strings
More messages converted
The last of the .format strings
Change tests so they show failures better.
Update a test for new quoting
Change the remaining imports of django.util.translation.ugettext to use util.markup.ugettext
Also provide a wrapper for ungettext
Remove an impossible and unneeded import
Fix broken video transcript links etc