Deleted applications

This is a first attempt at solving some of the issues raised in #11, as the Performance Platform will be removing applications in the coming months and we should tidy the release app as we do.

I've added a boolean to the application model and made a new table for when the boolean is true. I've set the URL for this to be `/applications/archive` but I'm open to suggestions about whether that's good or not. Applications are currently located at `/applications/:id`.