Django-loci

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- [deps] Added support for django 3.1
- [deps] Updated to openwisp-utils 0.6

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- [fix] Fixed websocket connect error for location change view
- [deps] Added support for Pillow~=7.2.0 & openwisp-utils~=0.5.1 and dropped their lower versions
- [deps] Added support for django-leaflet version 0.28

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- [fix] Fixed bug in floorplan fields
- [fix] Fixed bug which caused geographic map to disappears on narrow screens
- [fix] Fixed bug in JS logic
- [change] Allow to create an indoor location without specifying indoor coordinates

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- Added support to django 3.0, dropped support for django versions older than 2.2
- [admin] Fixed UX issue with ``is_mobile`` checkbox

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- Upgraded django-channels to version 2
- Upgraded dependencies (django, django-leaflet, Pillow)
- Geometry shouldn't be allowed to be None if not mobile
- Fixed admin fields hidden by mistake in case of validation errors
- Fixed type ``KeyError`` exception during form validation

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- [tests] Removed duplication of definition of floorplan test file