module changes for adding membership reminders, this commit adds
the full functionality. Groupwizard module now allows the 'co-op-day'
to be left unset and a date will be used that is one year away so
that buying can just stay open until quotas are met. The Groupwizard
will then update all timestamps when final dates are provided.
Invite module OpenBuyingGroups now uses a Purchase module method
OrderingAvailable which has been refactored. Organiser module checks
invite groups where required.