Add Accounts

* Account belongs to a User - the account owner. User belongs to an
account - account members.
* Accounts have a subdomain attribute. Requests to
accountSubdomain.host.com will show only posts from that account's
users.

* When a user signs up, they get an Account. In the future, will add an
account invite system.
* For now, the Account is used as a way to demonstrate subdomain logic.