Add migrations for user verification of building attributes

migrations/018.verification.down.sql

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+-- Building verification
+DROP TABLE IF EXISTS building_verification;

migrations/018.verification.up.sql

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+-- Building verification
+
+-- Users can verify the correctness of individual building attribute values.
+
+-- For a building, it's most useful to know the count of verifications of each
+-- attribute with current (or past) values.
+
+-- For a user, it's useful to show which attributes they have already verified
+-- on a given building.
+
+
+-- Store user-building-attribute verification
+CREATE TABLE IF NOT EXISTS building_verification (
+    verification_id serial PRIMARY KEY,
+    verification_timestamp TIMESTAMP default NOW(),
+    building_id integer REFERENCES buildings,
+    user_id uuid REFERENCES users,
+    attribute varchar, -- bit of a hack to refer to any `buildings` table column name
+    verified_value jsonb -- bit of a hack to include "any" value
+);
+CREATE INDEX building_verification_idx ON building_verification ( building_id );
+CREATE INDEX user_verification_idx ON building_verification ( user_id );
+CREATE INDEX building_user_verification_idx ON building_verification ( building_id, user_id );
+
+-- Enforce that a user only has one opinion about the correct value of an 
+-- attribute for a given building (don't need to allow multiple verified_values)
+ALTER TABLE building_verification ADD CONSTRAINT verify_building_attribute_once 
+    UNIQUE ( building_id, user_id, attribute );