set key length to 255 on mysql

schema/versions/mysql/137.sql

@@ -1,6 +1,6 @@
 begin;
 
-alter table ttrss_feeds add constraint ttrss_feeds_feed_url_owner_uid_key unique (feed_url(254), owner_uid);
+alter table ttrss_feeds add constraint ttrss_feeds_feed_url_owner_uid_key unique (feed_url(255), owner_uid);
 
 update ttrss_version set schema_version = 137;