Add mask option to highlight field

Many highlights have a corresponding mask (circle shape, heart shape, ...). The highlight action has currently no mask option built in. The corresponding masks should be able to applied automatically with a checkbox (BooleanField).

TODO
- make a list of which masks correspond to which highlight (if no mask corresponds the mask field will be ignored)
- migrate necessary functionality from the mask action to imtools so it can be used both by the mask and the highlight action
- add functionality to the highlight PIL function
- add a booleanfield 'Cut out with Mask' for the mask checkbox