fetchNextcloudApp: rewrite with fetchzip & applyPatches
fetchNextcloudApp: rewrite with fetchzip & applyPatches

There are the following issues with the current implementation:

* `fetchurl` with a tarball from GitHub appears to break occasionally
  because the tarballs are not necessarily reproducible. Because of
  that, `fetchFromGitHub` unpacks the tarball already because the
  contents are actually reproducible in contrast to the tarball. To have
  the same behavior here, we use `fetchzip` now (and `applyPatches` on
  top to apply additional patches if needed).

* Fixes the way how patches are applied. Previously, when having patches
  for a git checkout of the app, these wouldn't apply because the
  `appname-version` prefix is missing.

* Because all old hashes are broken with this, I added an evaluation
  check that breaks evaluation when using the old API (i.e. with
  `name`/`version` which are not needed anymore).

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