Refactor database

WARNING: migration will wipe most djsopnet_ tables

Remove user/create/edition times from purely sopnet tables
Change slice and segment hashes from strings to 64-bit integers
Slice and segment hashes are now PKs and FKs
Add multi-column unique constraints and indices to many tables
Change feature values to double precision
Rename FeatureNameInfo model/table to FeatureInfo
Add feature weights
Conflict sets are now related to exactly two slices
Remove ConflictSetRelation model/table
Remove SegmentCost model/table (catsop does not store costs)
Require segments to have at least one slice
Remove redundant db_index directives from foreign key columns