Allow config to specify some jobs as self enqueueing

[hlascelles]

We have a number of self-enqueueing jobs that we never want to delete, as they run forever.

We would like to specify that que-web should not delete these if a "Delete All" was pressed, or show an extra warning that that is about to happen if they are included. Also, the "single delete" (dustbin) image should be hidden or show an extra warning.

Additionally, a warning could be shown if they are missing.

For example, jobs like que-scheduler should always be present.

Would you be open to a PR for this?