Added Client object, Users.company, Projects.client

Adds a new Client object, which can hold users and projects. This is a great way of registering companies which assign your studio with projects.

Upon creation of a Client, you can (optionally) add users and projects.

A user can be created in stalker and be assigned to the Client. Also, a project can be created and be assigned to the Client.

Tests and Alembic revision included, although I found the removal of foreign keys a bit sketchy. Not sure if the Alembic revision containing the foreign key removals in `downgrade()` could be done in a better way. Ideally I would have wanted all foreign keys to be dropped when the column itself is dropped.