Add grid image content format and corresponding migrations

The model of grid image contents has been overhauled to store data more
efficient. Fields are now represented as a single index int value
instead of a semicolon separated String. Instead of storing all
selectable fields now only the correct ones are stored.

Further changes:
* Field size instead of scale factor and zoom level for grid
* Normalized values for offsets and field size
* A single scale factor value for the image
* Rotation in degrees
* Removed coloring settings