Add database logic to clear out requests that have 0 votes

Also, clean up some db constraints.

Fixes #29

db/upgrade/v0.6-v0.7.sql

@@ -0,0 +1,24 @@
+BEGIN;
+
+UPDATE version SET minor = 7;
+
+ALTER TABLE votes
+	ALTER COLUMN requestUrl SET NOT NULL
+	ALTER COLUMN userId SET NOT NULL;
+
+CREATE OR REPLACE PROCEDURE clear_zero_votes()
+	LANGUAGE SQL
+	AS $$
+		DELETE FROM requests WHERE NOT EXISTS
+			(SELECT FROM votes WHERE requests.url = votes.requesturl);
+	$$;
+
+CREATE OR REPLACE PROCEDURE delete_vote(url varchar,voteuser int)
+	LANGUAGE SQL
+	AS $$
+		DELETE FROM votes WHERE requesturl = url AND userid = voteuser;
+		CALL update_scores();
+		CALL clear_zero_votes();
+	$$;
+
+COMMIT;