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/models/user.rb

@@ -139,7 +139,7 @@ class User < ApplicationRecord
 
       # XXX downcasing is the wrong way to do case-insensitive comparison for unicode (should use casefolding).
       def find_by_name(name)
-        where("lower(name) = ?", normalize_name(name)).first
+        where_ilike(:name, normalize_name(name)).first
       end
 
       def normalize_name(name)