Wagtailvideos

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- Groundwork for pluggable backends added
- Video width/height now stored against the video model
- Removed enumchoicefield as a requirement (rip)

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- Changed wagtail.contrib.modeladmin to wagtail_modeladmin
- Removed references to wagtailadmin/shared/field_as_li.html
- Removed "created in" from wagtail admin

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- Upped minimum wagtail version to latest LTS (5.2)

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- Fixed bad migration

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- Upped min Wagtail version to 4.x
- Added support for limiting video choices based on collection permission

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- Added collection dropdown to chooser