Closes #228

This endpoint adds a quick way for clients to request arbitrary image
sizes based on the functionality available in the versatileimagefield
package.

If we have more advanced use cases we could either take a look at
integrating libvips [1] or delegate requests to a thumbor instance [2].

[1] https://github.com/libvips/pyvips
[2] https://github.com/thumbor/thumbor

Closes #203

This reverts commit ad526281.

Closes: #187

remove TimeSlotManager and use them as functions

Closes: #180

This is the counterpart to the
requires_express_permission_for_publication field of the License model.