Revert "FIX: when creating new PM username/groupname should be case-insensitive"

This reverts commit 2be79d94f5.

This is affecting multiple code path. Investigating.

lib/topic_creator.rb

@@ -211,7 +211,7 @@ class TopicCreator
     names = usernames.split(',').flatten
     len = 0
 
-    User.includes(:user_option).where(username_lower: names).find_each do |user|
+    User.includes(:user_option).where(username: names).find_each do |user|
       check_can_send_permission!(topic, user)
       @added_users << user
       topic.topic_allowed_users.build(user_id: user.id)