Fix timeouts in source:<url> searches and bookmarklet.

* Change the source index on posts from `(lower(source) gin_trgm_ops) WHERE source != ''`
  to just `(source gin_trgm_ops)`. The WHERE clause prevented the index
  from being used in source:<url> searches because we didn't specify
  the `source != ''` clause in the search itself. Excluding blank
  sources only saved a marginal amount of space anyway. This fixes
  timeouts in source:<url> searches and in the bookmarklet (since we do
  a source dupe check on the upload page too).

* Also switch from indexing `lower(name)` to `name` on pools and users.
  We don't need to lowercase the column because GIN indexes can be used
  with both LIKE and ILIKE queries.

app/logical/post_query_builder.rb

@@ -197,7 +197,7 @@ class PostQueryBuilder
 
     if q[:source]
       if q[:source] == "none"
-        relation = relation.where("posts.source = ''")
+        relation = relation.where_like(:source, '')
       else
         relation = relation.where_ilike(:source, q[:source].downcase + "*")
       end
@@ -205,7 +205,7 @@ class PostQueryBuilder
 
     if q[:source_neg]
       if q[:source_neg] == "none"
-        relation = relation.where("posts.source != ''")
+        relation = relation.where_not_like(:source, '')
       else
         relation = relation.where_not_ilike(:source, q[:source_neg].downcase + "*")
       end