Implement local authentication for the admin tool

[GUI]

Right now, to login to the API Umbrella admin tool, it currently relies on 3rd party login providers (like Google, GitHub, etc). One of the providers we enabled by default was Persona, which was nice, since it didn't require any oauth2 tokens or configuration on the part of the API Umbrella administrator. This let you login to the API Umbrella admin immediately after installing a stock API Umbrella installation without any further configuration.

However, with Persona going away (#313), it means we don't really have an easy option for logging into the admin without requiring the API Umbrella administrator to setup tokens with at least 1 authentication provider.

While I like not dealing with authentication, I think it might be worth implementing a simple local login option that API Umbrella admins could use. They could still disable this local login option if they'd prefer to rely only on 3rd parties, but having a local login option might be the lowest friction option for people looking to try API Umbrella. The need for a local login option has also come up previously for more firewalled environments that might not be able to utilize 3rd party logins (eg, #124, #247).

With our usage of Devise to handle logins, it actually shouldn't a huge lift to implement a very basic login mechanism, since Devise does most all of that for us. We mainly just need to enable Devise's default behavior around having local accounts.