Revert "[#19] Hide Link option from the upload field in resource form"

This reverts commit 8b549c338eca747dbcf42d4ded0a0db72ae5db89.
[#47] Support externally hosted files

Changed validators to require one or the other, added tests. Still not
clear how the requests must be sent in this case to the EC API.

TODO: handle actual request
TODO: Update mock EC