Solid-backend

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- Add media_object app as extension of photpgraph app.
- Fix the static filtering in the message app.
- Make the subject of the emails sent by the contact app dynamically add the PROJECT_NAME

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- Hotfix contact App.

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- Rename the Slideshow and SlideshowImage 'img' fields
- Add tests for relationship fields
- Create admin action that switches Deep Zoom image options
- Fix false declared 'required' attribute of some fields in swagger
- Fix swagger crash caused by a self-referential TreeNodeSerializer field

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- TreeNode fields are modified
- BaseProfile has an additional TreeForeignKey field to TreeNode
- TreeNodeSerializer is adapted and improved
- FieldMappingsEndpoint is removed
- Tests for BaseProfile and TreeNode are added
- ProfileEndpoint provides Photographs ordered by profile_position

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- SlideshowEndpoint provides only active objects ordered by position
- GlossaryEntryEndpoint provides objects ordered by term

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- Add ConcatCharField and FromToConcatField fields