Replace use of PowerConverter when we upgrade to Hyrax 3 #1962
Description
It looks like the PowerConverter class (it's a sipity / workflow thing) will be removed from Hyrax 3, per this slack thread: https://samvera.slack.com/archives/C0F9JQJDQ/p1571415688067000 and this PR (samvera/hyrax#4097)
We use PowerConverter extensively in the Laevigata application: https://github.com/curationexperts/laevigata/search?q=PowerConverter&unscoped_q=PowerConverter
When we upgrade Laevigata to Hyrax 3, we're going to need to re-write those bits.