Add an administration route to create invitations

[vxgmichel]

At the moment, invitations are created by:

• organization administrators for user or shamir invitations
• the user themselves for device invitation

We should also provide an administration route to let external services create new invitations.

This would be useful in two cases specifically:

• the creation of user invitations could be automated using a directory such as an LDAP
• the creation of shamir invitation could be automated through Parsec Auth when a user is trying to recover the access to an organization

Since the authenticated commands for creating invitations already exists, exposing those on the administration interface should be relatively simple.