Switch to using Devise for Authentication

[dannysmith]

Devise is pretty standard in the Rails world. It allows us to abstract some of the complexity of handling users and auth to the Gem.

Main advantages are:

• People will get it when using Jimmy as a base template for their app.
• It has plugins to handle login/signup with Twitter, Google, FB etc.
• It has plugins for 2FA with Authy, I expect.

It may be better to use Authlogic for simplicity.

Steps:

0. Check if Authlogic supports 2FA/Twitter/FB etc. Evaluate.
1. Install Devise
2. Generate the scaffolding and routes so they are in the codebase not the gem.
3. Edit these files so they fit in with Jimmy.
4. Enable login by Twitter / FB / GH and Google.
5. Enable 2FA somehow.
6. Clean the whole thing up and write some tests.
7. Invent some way to authenticate over API (using JWT etc) and to the React components.

[dannysmith]

Opened PR #198 which replaces the custom auth mechanism with Devise. I'm still not convinced that this is the right direction to take, since Devise has a whole lot of magic in it.