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

(take 2)

https://meta.discourse.org/t/case-sensitivity-in-links-to-groupname/147596
https://meta.discourse.org/t/remove-case-sensitive-in-adding-users-to-a-message/151275

app/controllers/posts_controller.rb

@@ -747,8 +747,8 @@ class PostsController < ApplicationController
     end
 
     if recipients
-      recipients = recipients.split(",")
-      groups = Group.messageable(current_user).where('name in (?)', recipients).pluck('name')
+      recipients = recipients.split(",").map(&:downcase)
+      groups = Group.messageable(current_user).where('lower(name) in (?)', recipients).pluck('name')
       recipients -= groups
       emails = recipients.select { |user| user.match(/@/) }
       recipients -= emails