add a button to clear the release lookup fields

[UltimateRiff]

when I'm adding multiple releases, I find it easiest to have my importer in one window and the artists' page in another, so I can just click and drag a link when moving to the next release. with how Harmony currently works, I've got to highlight the whole field and backspace before I can do this

an alternate option would be to clear the provider and GTIN fields at the top after looking up a release, but there might be a reason to show that even after the lookup. perhaps a second "new lookup" set of fields could work too? I'm up for any solutions~

[phw]

One way that is currently available for getting a blank form is clicking on the "Release Lookup" tab on the top. That works because the form does not remember the last state.