New GET /v1/invitations endpoint

[sergeychernyshev]

Create new GET /v1/invitations endpoint that returns a list of invitations current user sent (excluding admin invites, e.g. is_admin_invite is false)

Return schema:

• code - string (invitation code)
• status - ‘accepted’ | ‘sent’
• created - date invitation was created
• name - string (invited person’s name)
• invitation_email- string (invited person’s email)
• invitation_note - string (invitation note)
• account_id - integer (optional - if invited to join account)
• plan_slug - slug (slug string of subscription plan)