Brought codebase to v3.1

[ZhangTerrence]

I noticed that init.sql file in their Github was updated to v3.1 so I also updated our codebase to accomodate that.

Main Changes

• Made most of the relations bidirectional so inserting is easier.
• Changed toEntity methods because of the directional relation.
• Refactored attribute and metadata to their own separate service. Other entity services now calls those two services instead.
• I also finished implemented Location, Taxonomy, and TaxonomyTerm routes.

Insertion and deletion should work all the entities here. As for updating, so far I've only implemented it for non-relation attributes.