Allow locations to be in multiple groups and on multiple group maps

[k7hpn]

Currently:

1. A location cannot be added to multiple location groups (lines 48-49 in LocationGroupRepository.cs seem to restrict that).
2. If a location is in multiple location groups it can only have one as DisplayGroupId (that property is in the Location record).

Steps:

1. Fix LocationGroupRepository to allow a location to be in multiple groups properly.
2. Investigate removing the DisplayGroupId property from the Locations table to remove the 1-to-1 restriction and moving that relationship to a ShowOnMap property in LocationGroup. Does this break anything? There's no reason a location shouldn't be able to be in two maps.
3. Add the management of location groups to the new Locations management interface (under /Locations/ in Ops rather than the old interface under /SiteManagement/Locations/).