Allow Superusers to 'Remove' datasets

[jleighherzog]

There are instances when datasets should be removed from Dryad (vs withdrawn). For example, the option to "remove" would be selected for datasets that hadn't been submitted and will never, should never be reinstated. Select "withdraw" for submissions that have had a 'Submitted' status in the history.

It would be useful to allow only Superusers the option to "Remove dataset" from the Dryad Admin site. It's been suggested to include a button for removal next to the "Force Edit" feature. @ryscher: I think it will be critical to confirm when to remove vs withdraw so if you have other instances or ideas about when to select one vs the other, please chime in.

There's an existing, related ticket to allow users with the 'Curator' role to bypass any system requirements in order to successfully withdraw on behalf of authors or assist authors with their upload (see ticket).

[jleighherzog]

Another option would be to allow curators the ability to edit the status of an 'In Progress' submission to easily withdraw it. Currently, the little pencil icon isn't available.

[jleighherzog]

Related to new ticket: #2544 (comment)
The new ticket does not include the option to 'remove' submissions, only 'withdraw' from any status