Multiple-file upload from front end: way to easily resume with idempotency checks

[ermbutler]

Problem: If for some reason you are uploading of a set of multiple files but it fails before completion, the only way to check where it ended / restart is manual. Currently there are backend scripts we use that include checks for this, but they are not accessible via the front-end.

Solution: Idea from @jrtcppv -- we could add an upload dialog box that allows you to set options such as idempotency checks.

[jrtcppv]

Related issue: #374.

[bctcvai]

Related to: https://github.com/cvisionai/tator/issues/1153 as well.