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/models/application_request.rb

@@ -63,7 +63,7 @@ class ApplicationRequest < ActiveRecord::Base
     req_type_id = req_types[req_type]
 
     # a poor man's upsert
-    id = where(date: date, req_type: req_type_id).pluck(:id).first
+    id = where(date: date, req_type: req_type_id).pluck_first(:id)
     id ||= create!(date: date, req_type: req_type_id, count: 0).id
 
   rescue # primary key violation