Use param sanitiser in BatchInvitationUser
Use param sanitiser in BatchInvitationUser

As described in the initial UserParameterSanitiser commit
(https://github.com/alphagov/signonotron2/commit/23e5383fded6bab2a55e755e2477bbc779a279de)
we're moving to using role-based strong parameters-style sanitising everywhere.

The DeviseInvitable-provided `User.invite!` eventually creates a new User
record by way of mass-assignment, so from now on, anything we pass to that
method should be sanitised.

alphagov/signonotron2,alphagov/signonotron2,alphagov/signonotron2,alphagov/signonotron2