Invites

[joepio]

See atomicdata-dev/atomic-data-docs#23

• Add Invite Model and props to default store
• Use query param to add Agents to some resource using invites, powered by a plugin
• Check the rights of the Inviter on creating the invite (commit) Moved to Check rights of Invite creator #182
• Check the rights of the Inviter on redirecting the Invitee Moved to Check rights of Invite creator #182
• Check usagesLeft
• Decrease usagesLeft
• Update usedBy (edit: having doubts about this: it introduces a privacy risk, as others can know about who used the invite)