Solid-backend

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- Fix problem wher Phtograph --> MediaObject migration would run into `models.deletion.ProtectedError`

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- add tags to QuizSerializer
- SlideschowPagesEndpoint filter by show
- Implement filter by categories with django-filter in SlideshowEndpoint

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- Only show return Categories with at least one active Slideshow
- Rework SlideshowEndpoint, only retunr SlideshowPage ids in LIST, and full object in detail
- Provide Title Image for Categories
- Make it possible to have GlossaryEntries that have either a text or just Links
- Sort GlosarryEntries by Alphabet

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- Minor fixes to SlideShows