Allow upload-only jobs

[diamondap]

DART's internals currently support upload-only jobs, but it's impossible to create them through the UI, which insists on valid bagging parameters.

Upload-only jobs could be implemented like validation-only jobs: drag and drop items you want to upload, then tell DART where to send them.

The use cases here are:

1. Uploading existing bags created with external software.
2. Retrying uploads of jobs where bagging succeeded but upload failed. With very large bags, it would be nice to skip the bagging step and just retry the upload.

[diamondap]

This feature has been added to the latest internal build and should be available in the next public release.

[diamondap]

These features are available in version 2.1.0. For upload-only jobs, you can choose Jobs > Upload from the menu, then choose what you want to upload and where you want it to go.

For jobs where bagging has completed but the upload failed, if you click the green Run Job button again, you'll see a dialog asking whether DART should rebuild the bag and upload it, or just upload the existing copy of the bag.

See the release notes for version 2.1.0.

You can download the new release from https://aptrust.github.io/dart-docs/#download

[diamondap]

Re-opened because DART 2.1.0 is currently not viable. For now, we're back to the 2.0.22 release, which does not support this feature. The problem with 2.1.0 is #548