V3: SS4 support (beta)

Ss4 improvements (#50)

* Add a custom field-type `DBFocusPoint`. Move all focus point calculations into this class.
Remove unneeded InterventionBackend.
* Changed translation files to simply use the `db_FocusPoint` fieldname.
* Changed FocusPointField to behave more like a normal FormField.
* Renamed Test-class to open up naming possibilites for future test-classes :)
* Implement migration task and auto-migration.
* Added react component.
* Update to CSS and SVG for gradient & crosshair.
* Flipped Y-Coordinate to have consistent math (-1 top/left, 1 bottom/right).
* Added more unit-tests.
* Added documentation about the build-tools.