FEATURE: Mention @here to notify users in topic (#14900)

Use @here to mention all users that were allowed to topic directly or
through group, who liked topics or read the topic. Only first 10 users
will be notified.

lib/guardian.rb

@@ -541,6 +541,15 @@ class Guardian
     UserAuthToken.hash_token(token) if token
   end
 
+  def can_mention_here?
+    return false if SiteSetting.here_mention.blank?
+    return false if SiteSetting.max_here_mentioned < 1
+    return false if !authenticated?
+    return false if User.where(username_lower: SiteSetting.here_mention).exists?
+
+    @user.has_trust_level_or_staff?(SiteSetting.min_trust_level_for_here_mention)
+  end
+
   private
 
   def is_my_own?(obj)

lib/validators/not_username_validator.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class NotUsernameValidator
+  def initialize(opts = {})
+    @opts = opts
+  end
+
+  def valid_value?(val)
+    val.blank? || !User.where(username: val).exists?
+  end
+
+  def error_message
+    I18n.t('site_settings.errors.valid_username')
+  end
+end