Allow Invitation Email Translation

[a-angeles13]

Should address Issue #869

Added a generic invitation email template that uses jinja {% trans %} blocks to facilitate rendering without the render_localized_template function as suggested by almet.

I also modified the babel.cfg file to remove the jinja extensions as they are now included by default and were causing make update-translations to fail. I also modified some f-strings that were being used in conjunction with babel's gettext alias _() because this causes extraction errors with pybabel.

Modified budget_test.py to include a test of translated invite emails to ensure quality.

Language files in `ihatemoney/translations' should be ready to be populated with translations. Until then only English and French will be supported with unsupported languages defaulting to English.

[a-angeles13]

The check tests (3.7, minimal, postgresql; 3.7, minimal, mariadb; 3.9, minimal, sqlite; 3.10, minimal, sqlite) are failing due to depreciation in Werkzeug 2.1

Specifically, self.local.__ident_func__ in line 215 of <test_path>/werkzeug/local.py is now depreciated and will not work in Python versions 3.7+. This was noted in a warning on line 34 of test of 3.7, minimal, postgresql in a previously successful merge. link.

Copy of Warning excerpt mentioned above:
../home/runner/work/ihatemoney/ihatemoney/.tox/py/lib/python3.7/site-packages/werkzeug/local.py:215 /home/runner/work/ihatemoney/ihatemoney/.tox/py/lib/python3.7/site-packages/werkzeug/local.py:215: DeprecationWarning: '__ident_func__' is deprecated and will be removed in Werkzeug 2.1. It should not be used in Python 3.7+. return self._local.__ident_func__

I suggest that the merge check test suite be prioritized because any future pull requests will be blocked under any Werkzeug 2.1+.

Edit: Created new issue #1018

[Glandos]

Hi, thanks for taking care of this!
First, yes, the tests are failing, and it's not your fault, as far as I see.

Secondly, could you remove all modifications done to .mo/.po files? They are handled by Weblate, so we only need modifications in messages.pot

Finally, it's an open question: do we want to have a lot of sentences to translate? In my opinion, the whole invitation email is a block in itself, translating it into small pieces will not bring any reuse cases, and it can be harder for translators to find the right wording. It will generate a big box with a loooooong string, but on the other end, the template will be much simpler, with only one {%trans%} tag pair.

[almet]

Superseded by #1101. Thanks for your work, trying to get it integrated :-)