Implement drag and drop uploading of files

We have various drag and drop areas that currently don't work with drag and drop. A system should be implemented that allows all these areas to actually work, ideally sharing the same code.

@crwinfrey do we need drag and drop uploading for the Friday tests?

@grote Since we're mainly testing with the trainer types, no.

We may have some developers testing, so it would be nice. But depending on the dev progress, it should be considered low priority for the tests. I will provide designs in the case that you get to it.

changed milestone to %1.0

Thanks for the clarification. I changed the milestone.

I will provide designs in the case that you get to it.

We currently have this design from you and implemented it already:

If you want to change it, please reopen #33 (closed).

mentioned in issue #85 (closed)

mentioned in merge request !77 (merged)

mentioned in issue #84 (closed)

added dev label

added Doing and removed ~2112488 labels

removed Doing label