[DB] Fix Doctrine errors due to lack of column uniqueness

So, Doctrine doesn't like that `GSActorTag.tag` is not unique, even
though composite key `[tagger, tag]` is. `tag` can't unique, but
doctrine doesn't understand this. This seems like a Doctrine bug that
should be investigated. For now we'll just not mark it as a foreign
key

src/Entity/GSActor.php

@@ -296,7 +296,10 @@ class GSActor extends Entity
                 'modified'         => ['type' => 'timestamp', 'not null' => true, 'default' => 'CURRENT_TIMESTAMP', 'description' => 'date this record was modified'],
             ],
             'primary key' => ['id'],
-            'indexes'     => [
+            'unique keys' => [
+                'gsactor_nickname_uniq' => ['nickname'],
+            ],
+            'indexes' => [
                 'gsactor_nickname_idx' => ['nickname'],
             ],
             'fulltext indexes' => [