--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
New applications should use sass-rails 5.0
Mention that Action Mailer methods are not deferred [ci skip]

Closes #17965, #17968

[Godfrey Chan, Miklós Fazekas]
Add AC::Parameters#to_unsafe_h

As suggested in #16299([1]), this method should be a new public API for
retrieving unfiltered parameters from `ActionController::Parameters`
object, given that `Parameters#to_hash` will no longer work in Rails
5.0+ as we stop inheriting `Parameters` from `Hash`.

[1]: https://github.com/rails/rails/pull/16299#issuecomment-50220919
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
Merge pull request #18021 from nateberkopec/fix-filter-param-symbol-dup

ParameterFilter shouldn't try to dup symbol keys
Merge pull request #18022 from agrobbin/url-helpers-optional-scopes

Allow URL helpers to work with optional scopes
Conflicts:
	actionpack/CHANGELOG.md
Remove extra line from previous merge

Closes #18023
Fix handling of positional url helper arguments when format is false

There is no need to subtract one from the path_params size when there is
no format parameter because it is not present in the path_params array.

Fixes #17819.

(cherry picked from commit 1a50be823186082e8b5a29d8e9ea3a289318d9c1)

Conflicts:
	actionpack/CHANGELOG.md
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
autoloading guide: relax type of constant names [ci skip]
autoloading guide: new section about why Kernel#autoload is not involved [ci skip]
autoloading guide: Kernel#autoload -> Module#autoload [ci skip]
autoloading guide: minor edits.

/cc @fxn
autoloading guide: documents nesting