Add user invitations table and modify UserController

[dvsshadow]

Currently, the addUser functionality in the UserController adds a new user against their will, this should be changed and an invitation to the user should be made instead.
This implies adding a new table USER_INVITATIONS, and modifying the addUser handler.

[PTKay]

This should also target the web app, as we need to support invitations in there, and the case where a user does not accept the GitHub invites immediately. For example. we could have a bell on the top of the web app warning the user of pending invites, so they can accept the GitHub invites for both the organization and the repository.