Allow admin options to be set via environment variables

[owenfiscus]

First off, great project. I've been looking for something like this for ages and the Gitea OAuth integration is the icing on the cake. I would like to be able to set the settings listed in the Admin area of the application via environment variables.

For example, the Disable signup toggle could be set in a compose file as follows.

OG_DISABLE_SIGNUP: true

[thomiceli]

Hi, thank you! At first it behaved like this, but please see #5 #8