DEV: pluck_first

Doing .pluck(:column).first is a very common pattern in Discourse and in
most cases, a limit cause isn't being added. Instead of adding a limit
clause to all these callsites, this commit adds two new methods to
ActiveRecord::Relation:

pluck_first, equivalent to limit(1).pluck(*columns).first

and pluck_first! which, like other finder methods, raises an exception
when no record is found

app/helpers/application_helper.rb

@@ -127,7 +127,7 @@ module ApplicationHelper
 
     if current_user.present? &&
         current_user.primary_group_id &&
-        primary_group_name = Group.where(id: current_user.primary_group_id).pluck(:name).first
+        primary_group_name = Group.where(id: current_user.primary_group_id).pluck_first(:name)
       result << "primary-group-#{primary_group_name.downcase}"
     end