Interface for Users and additional user data

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The [users] section of the config file only has a key value pair (API Key -> database name). We need to be able to have more data associated with users.

1. We should have an interface for managing users (for example, func GetUserByAPIKey() User)
2. We should have a proper User interface. Right now that will only contain a function GetDatabase() but we will add more user fields in the future.
3. That interface should read data from a config file like it is today. In the future we'll want to use a database for managing user information.