Add functionality to cancel validation

[tvami]

Is your feature related to a problem?

Many times we realized that there was a mistake while the validation is running, or let's say we have 1000 prepIDs and 4 comes back with a problem that we know that will be an issue for the rest of them too. In these cases we have to wait for the validation of all of them to fail which is both time and computational power consuming.

Describe the solution you'd like

We request a functionality to cancel validation.

Current behavior

Waste 8*N hours (where N is the number of known bad prepIDs)

Expected behavior

If this functionality is added there would be another button which could be used to kill jobs on condor.

@ggonzr