Add media by URL

[anderm3]

Describe the solution you'd like
One the "Add a new media object" page in addition to "Select a File" it would be nice to have an option to import from a URL. I have existing photo hosting and downloading those images locally just to upload them again is a low and error prone process.

Describe alternatives you've considered
This would be in addition to existing methods this is the alternative.

Additional context
This could be considered a risk given that we would have the gramps-web process request data, but we are already allowing arbitrary uploads from users here so in that way the risk is the same.

[DavidMStraub]

This would first have to be implemented in the backend, so I'm moving the feature request there.

[hdholm]

Another consideration is that depending on where grampsweb is hosted, the URL may not resolve the same as on the client machine, e.g., local network file storage, or IP access controlled services. That could easily cause some confusion for some users.