Fix errors logged during installation

We had two problems:

1. `Redirect::to('/install')->send();` does not ends the execution, so the lines that follow are executed (so we had errors during the redirect to /install)
2. `Application->shutdown()` called `Database::get()`, which in turns try to open the default connection if it's not already open, so don't call `Database::get()` during the install process (this happened for every install step)