Kiwitcms-github-app

- Add GitHub issue-tracker integration which authenticates as the installed app. Fixes Issue 25
- Configure BugSystem for new repos. Fixes Issue 15
- Create Product & BugSystem records when installation_repositores change. Fixes Issue 21
- Trigger resync from GitHub via menu. Fixes Issue 19
- Trigger resync from GitHub after AppInstallation is configured. Fixes Issue 20
- Database: Add AppInstallation.settings_url field
- Link to the correct URL for GitHub settings. Fixes Issue 33
- Require user to be logged in for ApplicationEdit. Fixes Issue 36
- Update translation strings
- Add more tests

- Install settings overrides under ``tcms_settings_dir/``
(compatible with Kiwi TCMS v8.2 or later):

- does not need ``MIDDLEWARE`` and ``PUBLIC_VIEWS`` override anymore
- Remove ``GithubAppAuth`` backend, shipped with social-auth-core v3.3.0
- Fix a redirect to use the correct name of our social_core backend

- Do not fail if product already exists
- Do not fail if repository doesn't have description
- Search UserSocialAuth by uid and provider

- Address GitHub API deprecation not yet fixed in social-auth-core