CI/CD Clusters unable to detect already-installed apps, nor can it remove installed apps

After removing my cluster integration and re-adding it, I attempted to re-install Prometheus (since it didn't list it as "installed"), and it was met with the following error message because it had already been installed previously:

I think a few things should be done here:

1. These "install" actions should work in such a way that it does not fail when run a second time (the process should be idempotent)
2. GitLab should automatically detect when apps already exist in the cluster under the gitlab-managed-apps namespace
3. There should be a way to remove apps added via this integration. Currently I need to run kubectl delete namespace gitlab-managed-apps manually to reset this. We could put a button next to the "Remove cluster integration" that performs this command to remove the apps.
4. Removing a GitLab cluster integration should automatically remove the apps it had installed (or prompt the user with an option to do so)

/cc @joshlambert