Make AC::Params.permit_all_parameters thread safe

As discussed in #16299[1], this attribute is not thread safe and could
potentially create a security issue.

[1]: https://github.com/rails/rails/pull/16299#discussion_r15424533
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 #18003 from sikachu/permit_all_parameters-thread-safety

Make AC::Params.permit_all_parameters thread safe
Align the GemfileEntry API with stable branches

See 63462ec
Rails master requires arel master
New applications should use sass-rails 5.0
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]
Merge pull request #18006 from sikachu/add-params-to_unsafe_h

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

Use redis protocol
[guides] Fix method name inconsistency in a mailer example

[ci skip]
Merge pull request #18018 from yuki24/fix-method-name-inconsistency-in-mailer-example

[guides] Fix method name inconsistency in a mailer example
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.
Merge pull request #18020 from binaryberry/handle-positional-args-with-format-false

Fix handling of positional url helper arguments when format is false