[OEM-214] Fixed saving booking sort order when bookings have duplicate display_order
[OEM-214] Fixed setting of initial display_order when booking is created
[OEM-226] - initial hardcoded changes to admission letter
[OEM-226] - temp unit tests added for Letter Warning rule model.
[OEM-247] allow sessions to be unavailable past a fixed date
[OEM-226] - minor copy changes for all theatres
[OEM-226] - remove useless test
[OEM-226] - update release file
Merge branch 'master' into jira/OEM-247
+ update RELEASE file
Merge branch 'master' into develop