delete webapps from the database (bug 1020465)

fixes [bug 1020465](https://bugzilla.mozilla.org/show_bug.cgi?id=1020465)

WIP, do not merge/review
It's fine, it's fine ;)

We've just spent a whole lot of time to make sure that everything that should be "cascaded" was, and that none of the things that shouldn't, were.

In particular, we've double checked that Platforms and Tags were not cascade deleted, but ApplicationsVersions, Files, Versions (and their on-disk files) were.