Add Restart to Unfinished Batch Processing Actions

[kknoer]

This PR is in reference to this comment here: https://github.com/gdarko/wp-batch-processing/issues/16

I agree, a Restart option would be nice for longer batches in the event you make a code change to how a batch is processed and want to run the processing code against the entire batch from scratch, without waiting for the in-progress batch to complete.

I wasn't sure about how the buttons should display, so I added a new class for the Stop button styling specifically.