Fix layouts and rendering guide example using flash.now instead

Thanks @lintzchiang. [ci skip]
Merge pull request #11104 from drewda/patch-1

Adding additional plugins to debugging guide [ci skip]
`inspect` for AR model classes does not initiate a new connection.

Conflicts:
	activerecord/CHANGELOG.md
Fix rake test_sqlite3_mem

Clearly nobody uses this except me. It's fast people!
Fix test

Oops. We need to estalish/remove the connection in the setup/teardown,
else it messes with the fixtures.
Fix indentation
Apply default scope when joining associations.

For example:

    class Post < ActiveRecord::Base
      default_scope -> { where published: true }
    end

    class Comment
      belongs_to :post
    end

When calling `Comment.join(:post)`, we expect to receive only
comments on published posts, since that is the default scope for
posts.

Before this change, the default scope from `Post` was not applied,
so we'd get comments on unpublished posts.

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/lib/active_record/associations/join_dependency/join_association.rb
writing the new body can cause the response to be committed and the
request thread to return up the stack before the instance variable is
assigned.  Synchronize so that the ivar is assigned before the other
thread can activate.

fixes #10984
fetch value(s) from stringified options

Conflicts:
	actionview/CHANGELOG.md
Merge pull request #11178 from JonRowe/fix_tags_picking_string_keys_from_non_stringified_options_for_4-0-stable

Fetch value(s) from stringified options in tags that stringify options, based of 4-0-stable.
Merge pull request #11161 from dmitry/find_in_batches_works_without_logger

ActiveRecord find_in_batches should work without logger

When I set logger to nil both methods from Batches module find_in_batches or find_each should work anyway.
Merge pull request #11163 from nfedyashev/omit_turbolinks_on_javascript_skip_option

Omit turbolinks configuration completely on skip_javascript generator option
Conflicts:
	railties/CHANGELOG.md
Merge pull request #11175 from robin850/patch-4

Update a broken link into the fixtures' template [ci skip]
Merge pull request #11176 from schneems/schneems/plugin_new_help_fix

Fix `rails plugin --help`
Conflicts:
	railties/CHANGELOG.md
fix typo in ActiveModel::Error docs [ci skip]
Using preferred find_by syntax in guides [ci skip]
Adding find_by in guide to methods that trigger after_find [ci skip]
Remove a duplicated section [ci skip]

Attachments were previously covered so remove the useless part. Just
move a note about multipart headers set when the mail method is
triggered to the kept section.
backport: linking small rails 4 features image to a bigger one [ci skip]

View a bigger version onclick
https://github.com/rails/rails/issues/11100
Remove unneeded require