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

script/micro_bench.rb

@@ -15,11 +15,11 @@ Benchmark.ips do |b|
   end
 
   b.report("pluck with first") do
-    User.pluck(:name).first
+    User.pluck_first(:name)
   end
 
   b.report("pluck with limit") do
-    User.limit(1).pluck(:name).first
+    User.pluck_first(:name)
   end
 
   b.report("raw") do