feat(socialaccount): Add MediaWiki as a provider

[tgr]

Submitting Pull Requests

General

• Make sure you use semantic commit messages.
  Examples: "fix(google): Fixed foobar bug", "feat(accounts): Added foobar feature".
• All Python code must formatted using Black, and clean from pep8 and isort issues.
• JavaScript code should adhere to StandardJS.
• If your changes are significant, please update ChangeLog.rst.
• If your change is substantial, feel free to add yourself to AUTHORS.

Provider Specifics

In case you add a new provider:

• Make sure unit tests are available.
• Add an entry of your provider in test_settings.py::INSTALLED_APPS and docs/installation.rst::INSTALLED_APPS.
• Add documentation to docs/providers.rst.
• Add an entry to the list of supported providers over at docs/overview.rst.

[coveralls]

Coverage increased (+0.02%) to 93.653% when pulling c4faf77 on tgr:add-mediawiki-provider into 9f52b43 on pennersr:master.