Wagtailtrans

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- Add a `HIDE_TRANSLATION_TREES` setting with an edit in [language] button

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- Set cookie with language preference
- Check language cookie as fallback in the `TranslationMiddleware`

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- Added missing database migration

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- Dropped support for Python 2
- Dropped support for Wagtail versions earlier than 2.0
- Dropped support for Django versions earlier than 1.11
- Refactor .py files to line length 119
- Changed the version number to be inline with Wagtail

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- Dropped support for Wagtail versions earlier than 1.12
- Custom ``get_admin_display_title`` method now uses Wagtails ``draft_title``
- Fix: Get default language when ``languages_per_site`` is enabled now returns the correct default language for the site.

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- Added `appveyor` to the CI to ensure MSSQL compatibility
- Added a settings example file to the sandbox environment to ease the contribution setup
- Added documentation on ``Contributing to Wagtailtrans``
- Fix: language admin view throwing an exception with ``WAGTAILTRANS_LANGUAGES_PER_SITE`` set to ``True``