"Break out" of a frame from the server

Closes hotwired/turbo#257
Closes hotwired/turbo#397

Follow-up to:

* hotwired/turbo#257 (comment)
* hotwired/turbo#257 (comment)

Depends on hotwired/turbo#660

Introduces the `Turbo::Stream::Redirect` concern to override the
[redirect_to][] routing helper.

When called with a `turbo_frame:` option, the `redirect_to` helper with
check whether the request was made with the Turbo Stream `Accept:`
header. When it's absent, the response will redirect with a typical HTTP
status code and location. When present, the controller will respond with
a `<turbo-stream>` element that invokes [Turbo.visit($URL, { frame:
$TURBO_FRAME })][hotwired/turbo#649] where `$URL` is set to the
redirect's path or URL, and `$TURBO_FRAME` is set to the `turbo_frame:`
argument.

This enables server-side actions to navigate the entire page with a
`turbo_frame: "_top"` option. Incidentally, it also enables a frame
request to navigate _a different_ frame.

Typically, an HTTP that would result in a redirect nets two requests:
the first submission, then the subsequent GET request to follow the
redirect.

In the case of a "break out", the same number of requests are made: the
first submission, then the subsequent GET made by the `Turbo.visit`
call.

Once the `Turbo.visit` call is made, the script removes its ancestor
`<script>` by calling [document.currentScript.remove()][].

[redirect_to]: https://edgeapi.rubyonrails.org/classes/ActionController/Redirecting.html#method-i-redirect_to
[hotwired/turbo#649]: hotwired/turbo#649
[document.currentScript.remove()]: https://developer.mozilla.org/en-US/docs/Web/API/Document/currentScript

Gemfile

@@ -13,6 +13,7 @@ end
 gem "rails", rails_constraint
 gem "sprockets-rails"
 
+gem 'net-smtp'
 gem 'rake'
 gem 'byebug'
 gem 'puma'
@@ -23,6 +24,7 @@ end
 
 group :test do
   gem 'capybara'
+  gem 'capybara_accessible_selectors', github: 'citizensadvice/capybara_accessible_selectors', branch: 'main'
   gem 'rexml'
   gem 'selenium-webdriver'
   gem 'webdrivers'