Create an OrganizationInvite model

[joshsmith]

Problem

We need to have an OrganizationInvite model for inviting organizations.

Subtasks

• Create a migration
• Generate a short, unique code that can be used to fulfill the invite
• Email for inviting the person
• Organization title for use in sending an invite email and pre-populating the create form when the invite is consumed
• Afulfilled boolean that can only move from false to true
• Index on the code and email
• Write unit tests

[snewcomer]

@joshsmith looks like this is needed first?