Refactored tests to use a sample project.

Updated extraction:
* Removed special handling of single percent signs.
* When extracting messages from template text, doubled all percent signs
  so they are not interpreted by gettext as string format flags. All
  strings extracted by gettext, if containing a percent sign, will now
  be labeled "#, python-format".

Updated translation:
* Used "%%" for "%" in template text before calling gettext.
* Updated {% trans %} rendering to restore "%" from "%%".

refs ea27e26b

This mostly reverts 5643a3b5 and
81e1a35c.

Thanks Carl Meyer for review.

Ensured squashed migrations are recorded as applied when the
migrate command is run and all of the original migrations
have been previously applied.

Thanks Simon Charette for review.

As of b11564fd it's no longer
used elsewhere in Django and it isn't a documented public API.

Thanks Simon Charette for review.

Fixed #25241 -- Corrected ModelForm.save() error message when saving invalid form with UUIDField pk.

Fixed in f59fd15c

Added django.views.i18n.json_catalog() view, which returns a JSON
response containing translations, formats, and a plural expression
for the specified language.

Thanks Anssi Kääriäinen for the draft text and Simon Charette
for review.