Wagtailtrans

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- Make the dependency on `wagtail.core.middleware.SiteMiddleware` optional as it already can be removed in Wagtail 2.9

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- Added official support for Wagtail 2.9 and 2.10
- Make the permission `can_delete` compatible with Wagtail 2.10
- Make the page method `move` compatible with Wagtail 2.10
- Dropped support for Wagtail versions earlier than 2.7 (LTS)
- Replace deprecated ugettext, ugettext_lazy with gettext and gettext_lazy
- Replace deprecated force_text with force_str
- Fix: Languages in a Draft state result in failed routing from browser language

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- Dropped support for Wagtail versions earlier than 2.3
- Added support for Wagtail 2.8 and therefore Django 3.0

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- Added missing migration
- Removed Python 2 syntax

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- Add mention of wagtail.contrib.settings in the documentation
- Show languages in their local names.
- Add W504 to flake8 ignore, it's changed in a newer version of flake8
- Add env var WAGTAILTRANS_DISABLE_SIGNALS to disable many to many signal
- Add a disable_for_loaddata decorator on signals
- Add Simplified Chinese translation
- Validate default_language to not exist in other_languages

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- Fix: Add "?next=" to edit url to redirect back to the right tree after edit when `HIDE_TRANSLATION_TREES` is enabled.