Use a UNION for User.find_by_any_email

This is significantly faster than using a sub-query, at least when run
on the GitLab.com production database. The benchmarks are a lot slower
now with these changes, most likely due to PostgreSQL choosing a
different (and less efficient) plan based on the amount of data present
in the test database.

Thanks to @dlemstra for suggesting the use of a UNION.

app/models/user.rb

@@ -235,9 +235,18 @@ class User < ActiveRecord::Base
 
     # Find a User by their primary email or any associated secondary email
     def find_by_any_email(email)
-      User.reorder(nil).
-        where('id IN (SELECT user_id FROM emails WHERE email = :email) OR email = :email', email: email).
-        take
+      # Arel doesn't allow for chaining operations on union nodes, thus we have
+      # to write this query by hand. See the following issue for more info:
+      # https://github.com/rails/arel/issues/98.
+      sql = '(SELECT * FROM users WHERE email = :email
+      UNION
+      SELECT users.*
+      FROM emails
+      INNER JOIN users ON users.id = emails.user_id
+      WHERE emails.email = :email)
+      LIMIT 1;'
+
+      User.find_by_sql([sql, { email: email }]).first
     end
 
     def filter(filter_name)