fix: rm unique constraint on librepages_sites.repo_url

DESCRIPTION
    A repository might have several deployments. So unique repo_urls
    don't make any sense.

migrations/20220921122103_librepages_sites.sql

@@ -1,6 +1,6 @@
 CREATE TABLE IF NOT EXISTS librepages_sites (
     site_secret VARCHAR(32) NOT NULL UNIQUE,
-    repo_url VARCHAR(3000) NOT NULL UNIQUE,
+    repo_url VARCHAR(3000) NOT NULL,
     branch TEXT NOT NULL,
     hostname VARCHAR(3000) NOT NULL UNIQUE,
 	ID SERIAL PRIMARY KEY NOT NULL,