Swap habtm relations for slugs

Removes the has_and_belongs_to_many relationship from schemes and facets and replaces them with multiple indexed keys.
Clear facet to scheme relations
Merged master into multikeys branch with some manual conflict resolution
Merged purpose facet changes into multikeys branch
Add empty array default for facet fields
Add explicit index statements

Though Mongoid doesn't complain at the inline index attribute on a field,
the documentation only mentions this syntax on relations.
Remove indexes on HABTM relations
Only index locations
Remove legacy import data
Remove redundant facet tasks and data
Merge remote-tracking branch 'origin/multikeys'
Remove a couple of debugging puts.