Consolidate migrations

- Facilitate modifying migrations without cascading consequences for
  later migrations updating the same tables

db/migrations/00004_create_contracts_table.sql

@@ -0,0 +1,10 @@
+-- +goose Up
+CREATE TABLE watched_contracts
+(
+  contract_id   SERIAL PRIMARY KEY,
+  contract_abi  json,
+  contract_hash VARCHAR(66) UNIQUE
+);
+
+-- +goose Down
+DROP TABLE watched_contracts;