Do not overwrite subdomain when location was changed

* Install in subdomain 'test'
* Move to subdomain 'test2'
* Move to another existing subdomain 'www' (this should be detected as conflict)
* Move to subdomain 'www2' (this should not remove 'www'). This is why dnsRecordId exists.

migrations/schema.sql

@@ -60,7 +60,7 @@ CREATE TABLE IF NOT EXISTS apps(
     manifestJson TEXT,
     httpPort INTEGER,                        // this is the nginx proxy port and not manifest.httpPort
     location VARCHAR(128) NOT NULL UNIQUE,
-    dnsRecordId VARCHAR(512), // tracks any id that we got back to track dns updates (unused)
+    dnsRecordId VARCHAR(512), // tracks any id that we got back to track dns updates
     accessRestrictionJson TEXT, // { users: [ ], groups: [ ] }
     createdAt TIMESTAMP(2) NOT NULL DEFAULT CURRENT_TIMESTAMP,
     memoryLimit BIGINT DEFAULT 0,