Allow download of metadata/schemas

[ThomasJejkal]

Is your feature request related to a problem? Please describe.
Similiar to #32 it was also requested to allow downloading metadata schemas and documents in the metadata-management UI. Currently, this is only possible via copy&paste from the editor view, but direct download would be helpful for the user.

Describe the solution you'd like
Downloading schema/metadata can be implemented either in the schema/metadata view modal or directly in the listing table. Probably, the better solution would be the first, as the listing may not provide enough information to decide whether the list entry is what the user wants. In addition, putting the download in the view modal will also allow to download previous versions.

[Pfeil]

The download/reference from the search panel does also not work, at least on our demo instance. The URL is of form https://demo.datamanager.kit.edu/my-metadata. Not sure if it is worth a separate issue? Could also be a reverse proxy issue or so. Actually, I am not sure if the link is supposed to trigger a download or reference to the editing/viewing panel in the gui.

For illustration, I mean the reference in this view (example):