Run chown on startup, not when generating config

Synapse needs its data folder owned by the correct user to function.
Currently, this is achieved by running `chown` when generating the `homeserver.yaml` file only.

The issue with this is, it does not work for deployments where the container generating that config file is not the actual container running synapse later on. For example in my case I generated the config file on my local machine, then uploaded everything to a server and deployed synapse on there. And because chown never happened on that server, synapse was unable to write to the data directory.

Therefore I think it would be best to run chown on startup of the actual synapse service, NOT when generating the config file;
Either only on first startup (if that can already be detected), or with a flag to disable it; since running chown on very large directories can add quite some time to the service startup.