Support for sending PMs to email addresses (#4988)

* Added support for sending PMs to email addresses.

* Made changes after review.

* Added settings validator.

* Fixed tests.

app/controllers/posts_controller.rb

@@ -617,7 +617,10 @@ class PostsController < ApplicationController
       usernames = usernames.split(",")
       groups = Group.mentionable(current_user).where('name in (?)', usernames).pluck('name')
       usernames -= groups
+      emails = usernames.select { |user| user.match(/@/) }
+      usernames -= emails
       result[:target_usernames] = usernames.join(",")
+      result[:target_emails] = emails.join(",")
       result[:target_group_names] = groups.join(",")
     end