Rework the invitations system to allow more flexibility

[vincentcourtois]

Context

The current invitations system works fine but has three main limitations :

• it does not support the historization of what happened to the invitation
• it does not support extensibility well, as the possible statuses are a hard-coded enumeration, and nothing else than statuses can be stored
• it does not support custom data added to know for example who accepted your invitation

User story

As a Connected user I want to Access the history of the events concerning me in a campaign So that I can have all the informations about my own membership through time

Needs

We need a system that can :

• keep track of the modifications of the invitation through time
• be extended with new feature (eg an "honorific title" feature that could allow the addition of honorific titles to the invitation)
• receive additional data to enrich these events.

Proof of completion

• Code is developed
• Unit tests are in place
• The newly produced code is Rubocopified

[vincentcourtois]

This pull requests fixes historization : https://github.com/virtuatable/arkaan/pull/53