Merge pull request #17988 from caike/master

Add guides for `required` model generator option
Merge pull request #17970 from ulissesalmeida/foreign-type-has-many-has-one

Add foreign_type option for polymorphic has_one and has_many.
Conflicts:
	activerecord/CHANGELOG.md
No need to point to coffee-rails master when generating a edge app
Merge pull request #17793 from kamipo/fix_undesirable_range_error

Fix undesirable RangeError by Type::Integer. Add Type::UnsignedInteger.
Merge pull request #18000 from caike/master

Update guides with info about vendor/assets [ci skip]
Merge pull request #17994 from mfazekas/pluck-bind-values-master

Fix ProtocolViolation/bind message for polymorphic + pluck or group+calc
Improve the test case introduced by bd0d47e
Merge pull request #18010 from audionerd/patch-1

Fix default queue name prefix delimiter
Merge pull request #18003 from sikachu/permit_all_parameters-thread-safety

Make AC::Params.permit_all_parameters thread safe
--edge on 4-2-stable should track the 4-2-stable branch

See 24e1fff
Pass through the `prepend` option to `AS::Callback`

I'm not sure what's the use case for this, but apparently it broke some apps.
Since it was not the intended result from #16210 I fixed it to not raise an
exception anymore. However, I didn't add documentation for it because I don't
know if this should be officially supported without knowing how it's meant to
be used.

In general, validations should be side-effect-free (other than adding to the
error message to `@errors`). Order-dependent validations seems like a bad idea.

Fixes #18002
Mention that Action Mailer methods are not deferred [ci skip]

Closes #17965, #17968

[Godfrey Chan, Miklós Fazekas]
New applications should use sass-rails 5.0
Merge pull request #18017 from sikachu/4-2-stable-add-params-to_unsafe_h

Add AC::Parameters#to_unsafe_h
redis 3.2.0 doesn't allow tcp protocol anymore

Use redis protocol
Preparing for 4.2.0.rc3 release
new guide: constant autoloading and reloading
registers the autoloading guide in the CHANGELONG and release notes
Merge pull request #18030 from zzak/doc-timestamps-null-option-from-generator

Add `null` option to timestamps as migration generator does [ci skip]
autoloading guide, fix typo `Amin` -> `Admin`. [ci skip]

/cc @fxn