Allow users to self-host a learnawesome instance

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.