users: Move first-run steps into setup

Currently, users module has an extra setup step that is done during first-run: https://github.com/freedombox/Plinth/blob/master/data/usr/lib/freedombox/first-run.d/50_ldap

However, we plan to skip the module setups during image build (#875 (closed)), and have plinth do the setups the first time it runs. This can cause an issue, because the first-run may actually run before the setup.

Move the current first-run actions into the normal users module setup.