Add InvitedUser model, policy, and controller

[joshsmith]

Problem

We want to add an InvitedUser with the following attributes:

• email - string; required
• role - string; optional
• name - string; optional

And the following relationships:

• invitee - optional; eventually set to the user created from this invite
• inviter - required; the user who created the invite
• project - optional; the project the invite is for

We’ll need create, update, index and show endpoints.

If inviting to a project, the policy should check permissions similar to how ProjectUser works.

When the invite flow is complete, ie to claim an invite, we’ll need to:

• create a user
• set the invited_user’s invitee_id
• create a project_user for the project with the specified role

It may make sense to do this on the user create endpoint by specifying the InvitedUser id as a relationship.

[theochino]

Sorry for adding to this bug. Trying to get the attention of @joshsmith regarding another repo but all the emails are bouncing. @joshsmith could you reach at me?