Allow users to self-host a learnawesome instance

[nileshtrivedi]

We currently use Auth0 for login/signup, which is typically tied to a specific domain. The domain is also hard-coded in a bunch of places - including the webextension, mobile app and the crawlers which keep importing expert reviews about learning resources in the repository.

But we intend to support self-hosting after all. In fact, that was part of the idea why I implemented support for the ActivityPub protocol.

See: https://github.com/learn-awesome/webextension/issues/6

[nileshtrivedi]

Some goals for self-hosting:

• Using ActivityPub, all instances should be able to sync with each other. This means that LearnAwesome instances should act not just as a source, but also a sink for AcitivityPub events
• Write an article on how to configure an Auth0 account with various social logins with the correct permission scopes. Or alternatively, implement a different mechanism for authentication
• User, Topic, Item references will have to be namespaced. For eg: it won't be sufficient to address a user by alone. Instead things like reviews should be referenced to (, ) for them to be globally unique.
• Webextension and mobile app will have to offer a way to change the instance address, rather than hard-coding to learnawesome.org