Duplicate database index on nodes_tags

The database schema for Razor sets up two indices on nodes_tags that are the same thing: since `002_create_base_schema` specifies `(node_id, tag_id)` as the pk for the table, the additional index on `(node_id, tag_id)` is redundant (the pk forces creation of such an index already)

It might be useful for some queries to have the reverse index on `(tag_id, node_id)` instead.

In Scope

• Delete the duplicate "nodes_tags" index
• Create a new index called "tags_nodes" on the reverse order of the index (tag_id, node_id)

Current output:

razor=# \d nodes_tags

   Table "public.nodes_tags"

 Column  |  Type   | Modifiers 

---------+---------+-----------

 node_id | integer | not null

 tag_id  | integer | not null

Indexes:

    "nodes_tags_pkey" PRIMARY KEY, btree (node_id, tag_id)

    "nodes_tags_node_id_tag_id_index" btree (node_id, tag_id)

Foreign-key constraints:

    "nodes_tags_node_id_fkey" FOREIGN KEY (node_id) REFERENCES nodes(id) ON DELETE CASCADE

    "nodes_tags_tag_id_fkey" FOREIGN KEY (tag_id) REFERENCES tags(id) ON DELETE CASCADE