search: fix invalid username searches returning wrong results.

Partial fix for #4389.

* Fix invalid username searches returning all posts instead of no posts.
* Fix "user:A user:B" returning results for user:B instead of no results.
* Fix "approver:A approver:B" returning results for approver:B instead of no results.
* Add support for negated -commenter, -noter, -noteupdater, -upvote, -downvote metatags.
* Add support for "any" and "none" values for all username metatags,
  including negated metatags that didn't support "any" or "none" before.
* Change noter:any and commenter:any to include posts with deleted notes
  or comments. Note that commenter:<username> already included deleted
  comments before. This is so that commenter:any has the same behavior
  as commenter:<username>

app/models/user.rb

@@ -145,9 +145,14 @@ class User < ApplicationRecord
         find_by_name(name).try(:id)
       end
 
-      # XXX downcasing is the wrong way to do case-insensitive comparison for unicode (should use casefolding).
+      # XXX should casefold instead of lowercasing.
+      # XXX using lower(name) instead of ilike so we can use the index.
+      def name_matches(name)
+        where("lower(name) = ?", normalize_name(name)).limit(1)
+      end
+
       def find_by_name(name)
-        where_iequals(:name, normalize_name(name)).first
+        name_matches(name).first
       end
 
       def normalize_name(name)