Only create ltree postgres extension in migration if doesn't already exist (#3092)

migrations/2022-07-07-182650_comment_ltrees/up.sql

@@ -30,7 +30,7 @@ drop view comment_alias_1;
 
 alter table comment drop column read;
 
-create extension ltree;
+create extension if not exists ltree;
 
 alter table comment add column path ltree not null default '0';
 alter table comment_aggregates add column child_count integer not null default 0;