Relax uniqueness of Token.

- Doing this so that multiple users can have api test tokens.
- Not sure the app needs to be checking uniqueness of API tokens
  anyways.
Remove Plaid Class based sandbox control.

- Use env variables to let Plaid know to run itself in sandbox mode.
- I.E., `PLAID_SANDBOX=true rails server` and the initializers will
  handle the rest.
Refactor available institutions behavior.
Merge pull request #22 from jcdavison/plaid-user-auth-changes

Plaid user auth changes