[WIP] Fix hard dependency on gd

Just to start somewhere "setting the dimensions" is moved to the MediaManager. We might want to do it different now or later, but then at least it is removed from the model.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | no - todo
| Fixed tickets | #71 
| License       | MIT
| Doc PR        | N/A