Creates a new invitation into a Project. The invitation will be sent to the email address provided.
Invitation will also add the user to the parent workspace with the role reader.
If the invitation is already pending or expired for this email/project combo,
if will be updated with the new role and resend.

An email that the directory of the workspace's organisation manages, of a
person who is not a member of the workspace, gets no invitation: its item has
error_code invitation_directory_user. An organisation admin adds the person
to the workspace first. A member of the workspace is invited as usual.