feat: Add optional config of database connection pooling

.env.sample

@@ -12,6 +12,8 @@ UTILS_SECRET=generate_a_new_key
 # should work out of the box.
 DATABASE_URL=postgres://user:pass@localhost:5432/outline
 DATABASE_URL_TEST=postgres://user:pass@localhost:5432/outline-test
+DATABASE_CONNECTION_POOL_MIN=
+DATABASE_CONNECTION_POOL_MAX=
 # Uncomment this to disable SSL for connecting to Postgres
 # PGSSLMODE=disable
 REDIS_URL=redis://localhost:6379