Cannot drag & drop multiple files in a single request (to start a new submission)

[tdonohue]

Describe the bug
This bug was discovered during review of #724. That PR currently works around this bug as it throws a friendly error message if anyone attempts to upload multiple files at once.

The ng2-file-upload library currently has a limitation where it doesn't allow making a POST with multiple files.

• There's an open issue ticket with ng2-file-upload which describes a number of ways others have been able to workaround this problem: Upload multiple files in one request valor-software/ng2-file-upload#671
• There's also a (very old) PR to fix the problem: Multi-upload in a single request valor-software/ng2-file-upload#993 (but it was never merged).

The ongoing maintenance of ng2-file-upload seems unclear as it has not received updates for almost a year now. So, another option would be to search for an alternative upload library which better supports our needs.

Possible Alternative Library:

• https://github.com/pIvan/file-upload - Has had recent updates, and from the docs/demo seems to support multiple file upload.

[atarix83]

estimation for this task is about 40 hours

[tdonohue]

As discussed in today's meeting, this ticket will be delayed for 7.1.

@atarix83 : I realized we already have a "quick fix" for this ticket in place, as the original PR #724 simply shows an error if a user attempts to upload multiple files at once to start a new submission from the MyDSpace page. So, there's no need for you to investigate an additional quick fix here.