Allow superusers to bypass dataset file size limit

[ahamelers]

The Dryad size limit is 300GB, but sometimes there are legitimate reasons why a dataset must go above this. Currently, a dev team member must temporarily suspend the limit for all datasets, or use the backend to submit the dataset, in order for these large datasets to be submitted. Allowing superusers to submit datasets of any size would decrease the reliance on the dev team in these cases and remove the need to temporarily allow over-the-limit uploads for all datasets.

[ahamelers]

Related to #2689.

I think this would be fine to go ahead and do, but there needs to be something like overage fees for our members before we start allowing special case datasets of anything like 1TB, and some precise policy of what exactly is allowed in the meantime.