Fix broken link to Upgrading Ruby on Rails Guide

in the 4.2 Release Notes [skip ci]

Change to link suggested by @zzak [skip ci]
Merge pull request #16545 from jonatack/patch-9

Fix broken link to Upgrading Ruby on Rails Guide [ci skip]
Fix after_commit warning message
Merge pull request #16548 from arthurnn/fix_warn_message

Fix after_commit warning message
Deprecate TagAssertion instead of removing
Removed warning
Point to unreleased deprecated_sanitizer
Remove to_s from reflection.type in add_constraints

The instance var is already saved as a string in the initialization
method of AssociationReflection.

See https://github.com/rails/rails/blob/master/activerecord/lib/active_record/reflection.rb#L273
Merge pull request #16550 from eileencodes/remove-unnecessary-to_s-from-add_constraints-method

Remove to_s from reflection.type in add_constraints
Merge pull request #16542 from seamusabshere/numerify-pool-checkout-timeout-from-urls-4-1-stable

Make sure :checkout_timeout and :dead_connection_timeout are numbers
Conflicts:
	activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
require 'test_help' -> require 'rails/test_help'
Add missing require
Mention deprecation of assert_select and friends in the release notes [ci skip]
Sync Action View release notes [ci skip]
Sync Action Mailer release notes [ci skip]
Allow sending all emails through activejob

Added config.action_mailer.deliver_all_emails_later boolean flag and #deliver_now / #deliver_now!
methods. When #deliver / #deliver! is called, based on the value of deliver_all_emails_later
it will call #deliver_now / #deliver_now! or #deliver_later / #deliver_later!.