Bug 12478: change the schema for storing mappings

Bug 12478: change the schema for storing mappings

This makes the mapping schema a proper many-to-many relational
structure, which is a little more fiddly to work with, but will make
things like editors for it easier to write.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jesse Weaver <jweaver@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>