Add .env boolean variables to disable login options.

[0x0f0f0f]

It would be really useful to add these optional env variables to disable/enable certain login options.

ENABLE_GUEST_LOGIN=false # Set to true by default
ENABLE_GOOGLE_LOGIN=false # Set to true by default
ENABLE_TWITTER_LOGIN=false # Set to true by default

The app should crash if no login options are enabled, and the app should ignore the API keys and secrets variables if that login option is disabled

[markusenglund]

I don't really understand what this does. Could you explain a bit more why this would be useful?

[0x0f0f0f]

Let's say some of my coworkers use Google and don't have any Twitter account or viceversa, I shouldn't have to register an API key in the app 😄. On a local server with limited resources (such as my Raspberry Pi) it would also be useful to deny guest users to sign up and use valuable disk space

[markusenglund]

Ok, makes sense. If you want to make a PR I'll merge it.