Feincms3-sites

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- Verified compatibility with Django 3.0.
- Replaced ``ugettext*`` with ``gettext*``.

~~~~~~~~~~~~~~~~~~~

- Removed the requirement to anchor site regular expressions at the
beginning (meaning that e.g. ``example\.com$`` now works as it
should).
- Fixed a possible path uniqueness problem with descendants with static
paths.

~~~~~~~~~~~~~~~~~~~

- Removed ``apps_urlconf_for_site`` and ``apps_middleware`` since
feincms3's ``apps_middleware`` now automatically does the right thing
when used after ``site_middleware``.
- Made the ``site`` argument to ``AbstractPage.objects.active()``
keyword-only.

~~~~~~~~~~~~~~~~~~~

- Added an ``is_active`` flag to sites.
- Removed the check that only one site is the default, making it
possible to change the default through the admin interface.
- Made it possible to edit ``is_active`` and ``is_default`` through the
changelist.
- Updated the Travis CI configuration to cover a greater range of
Python and Django version combinations.

~~~~~~~~~~~~~~~~~~~

- Added validation of the ``host_re`` so that invalid input is catched
early.
- Sort non-default sites by host in the administration interface.
- Added ordering by ``position`` to the abstract page (necessary for
newer versions of django-tree-queries).
- Added support for using feincms3-sites without explicitly specifying
the site everywhere by taking advantage of the upcoming `contextvars
<https://docs.python.org/3/library/contextvars.html>`__ module and its
backports.

~~~~~~~~~~~~~~~~~~~

- Raised test coverage to 100% again.
- Added the possibility to define a default language per site.
- Switched the preferred quote to ``"`` and started using `black
<https://pypi.org/project/black/>`_ to automatically format Python
code.