This changes the default behaviour when a frame response is missing its
expected `<turbo-frame>` element.

Previously, when the response was missing its frame, we would trigger a
`turbo:frame-missing` event, and then (provided that event wasn't
cancelled) perform a full page visit to the requested URL.

However there are cases where the full reload makes things worse:

- If the frame contents were non-critical, reloading the page can turn a
  minor bug into a major one.
- It can mask some bugs where frames were intend to explicitly navigate
  out of the frame (`target="_top"`), by incurring a second request that
  loads the page that makes it seem as if it's working corrects.
- It leaves the user at a URL that may never be capable of rendering a
  valid response (since that URL was only intended to serve a particular
  frame). That means refreshing the page is no help in getting back to a
  working state.
- It can lose other temporary state on a page, like form values.

With this change, we no longer perform the full page visit. Instead, we
handle a missing frame by doing two things:

- Write a short error message into the frame, so that the problem is
  visible on the page.
- Throw an exception, which should make the problem quite obvious in
  development, and which allows it to be easily gathered by exception
  monitoring tools in production.

We keep the `turbo:frame-missing` event exactly as before, so
applications can still hook in to perform alternative behaviour if they
want.

Also remove `invalidate` that has effectively been replaced.

When a frame response is missing its expected `turbo-frame` element, we
consider it an error. Normally this is what we want.

In certain cases, though, it's better if a response without a matching
frame is treated as a full page instead. The classic example of this is
a login page being rendered in response to an expired session.

To allow this, we provide a way for a page to mark itself as
"breakoutable" by including a specific meta tag in its head:

    <meta name="turbo-frame-missing" content="visit">

When a response is missing its expected frame, but includes that meta
tag, we'll perform a full-page visit to it, rather than throw an error.

Rather than use a meta tag in the response, we can put a central list of
"breakoutable" paths in a single meta tag, and include that on
requesting pages.

Assuming that an application will have a small number of these paths, it
might be simpler to centralise them in one place and include that in a
shared layout.