RFC: Bounded Contexts for CodeCorps API and Phoenix 1.3+

# Problem

#749 deals with upgrading to phoenix 1.3

However, part of that upgrade is the encouragement to switch from a flat layer of models/schemas into a concept of bounded contexts

This would facilitate management, maintainability and understanding of the code, but requires some research and discussion before it can be attempted.

I'm already doing some reading, but I would not feel confident in doing this alone, so any ideas are welcome.

# Some links to go through:

- https://martinfowler.com/bliki/BoundedContext.html
- https://robots.thoughtbot.com/lessons-from-using-phoenix-1-3

# Initial ideas for boundaries

- `CodeCorps.Account` - creation of accounts, signing in, reseting password, etc, etc
- probably `UserRole`, `UserSkill`, `UserCategory`, etc, would go here to

- `CodeCorps.Forum/ProjectManagement` - Projects, Tasks, Comments, Memberships, etc
- probably `ProjectRole`, `ProjectSkill`, `ProjectCategory`, `TaskSkill`, would go here

- `CodeCorps.Funding/.Stripe/.Payments/.Subscriptions` - Stripe/payment stuff
- `CodeCorps.GithubIntegration`