Add (extensible) enum for license suggestions

[NicolasLooschen]

It would be good if we have a set of licenses that should be supported when uploading.

Mainly the different variants of the CC license would come to my mind. Which other licenses for image data exist (MIT, Apache, etc. is more for source code?) is somewhat beyond my knowledge.

@matthias-mueller @Zazi do you already have an opinion on this?

[matthias-mueller]

This is what we know so far:

1. There will be a list of well-known licenses that occur frequently
2. We must support cases, where a license is not in this list

UI wise we should have a free-text field that can has a license dropdown / picker feature so that the well-known licenses do not have to be entered manually.

Starting with the CC licenses is okay. Let's use the latest version and extend as needed. (the other licenses are indeed more for code)