Generalise sign-in methods into optional plugins

[brendanarnold]

Twitter and Facebook are hardcoded into the pol.is codebase as the only 3rd party sign-in methods. These should be abstracted out into plugins so that they can be removed entirely from a deployment.

• Simplifies cookie policy
• Untangles 3rd party code from codebase
• Better architecture for other sign-in integrations such as GovUK Sign-In

[patcon]

Love this! A generalized oauth2 flow might be a good start to work out how best to do this 🤔

Potential todo: cross-ref related issues in upstream polis repo, as this has def been discussed.