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

lib/validators/max_username_length_validator.rb

@@ -7,7 +7,7 @@ class MaxUsernameLengthValidator
 
   def valid_value?(value)
     return false if value < SiteSetting.min_username_length
-    @username = User.where('length(username) > ?', value).pluck(:username).first
+    @username = User.where('length(username) > ?', value).pluck_first(:username)
     @username.blank?
   end
 

lib/validators/min_username_length_validator.rb

@@ -7,7 +7,7 @@ class MinUsernameLengthValidator
 
   def valid_value?(value)
     return false if value > SiteSetting.max_username_length
-    @username = User.where('length(username) < ?', value).pluck(:username).first
+    @username = User.where('length(username) < ?', value).pluck_first(:username)
     @username.blank?
   end