5716 edit file api perms

[sekmiller]

What this PR does / why we need it:
This PR restricts the use of the import dataset and create dataset (with files) to accounts with superuser permissions. This is needed to strictly limit the instances in which file metadata/datafile records are created in the absence of a physical file.

Which issue(s) this PR closes:

Closes #5716

Special notes for your reviewer:

Suggestions on how to test this:
verify that a non-super user may still use the create dataset api as long as files are not included.

Does this PR introduce a user interface change?:
No
Is there a release notes update needed for this change?:

Additional documentation:

[coveralls]

Coverage decreased (-0.0005%) to 19.629% when pulling d64e8ce on 5716-edit-file-api-perms into 68bb656 on develop.

[landreev]

OK, I'm impressed that you added that whole dedicated test to api/DatasetsIT, just to get a BAD_REQUEST in the end... I mean, it makes sense, but still.

[sekmiller]

@landreev I thought there was going to be more to it than that, but after writing the test I realized that the update version has never been used to add files so I left it at that.

[landreev]

I thought there was going to be more to it than that, but after writing the test I realized that the update version has never been used to add files so I left it at that.

Makes sense. And it's still a useful test.