Decouple codidact from code

[Taeir]

This pull request decouples the codidact network from a lot of the pages. It does so by defining a helper function which checks the domain of the community for .codidact.[com|org]. It then disables or changes components on pages based on this setting.

TODO

• Special config file for these settings that do not change often or at all?
• Email addresses as settings
• Figure out something for the copyright in the mailer and devise mailer (layout)
• Network name setting?

Fixes #826

[cellio]

Thanks for working on this! I knew we only got some of them (the easiest ones, naturally) in the previous sweep, and then I lost track of it.

[Taeir]

I wonder whether the Redis cache is going to be faster than checking the string twice. It might be premature optimization.