Adding organisation to BatchInvitations

Organisation Admins can only batch invite users to join
an organisation within their organisation subtree.
Assigning user's organisation based on BatchInvite
Add organisation to Batch Invitation form
Just test code clean-up
All admins can invite users into organisations
Minor refactoring to remove view logic
Fixes needed after rebase
Store the organisation to assign invitee to
Merge pull request #135 from alphagov/152-batch-invitation-for-organisation-admins

Batch invitations for organisation admins