fix($compile): connect transclude scopes to their containing scope to prevent memory leaks

Transcluded scopes are now connected to the scope in which they are created
via their `$parent` property. This means that they will be automatically destroyed
when their "containing" scope is destroyed, without having to resort to listening
for a `$destroy` event on various DOM elements or other scopes.

Previously, transclude scope not only inherited prototypically from the scope from
which they were transcluded but they were also still owned by that "outer" scope.
This meant that there were scenarios where the "real" container scope/element was
destroyed but the transclude scope was not, leading to memory leaks.

The original strategy for dealing with this was to attach a `$destroy` event handler
to the DOM elements in the transcluded content, so that if the elements were removed
from the DOM then their associated transcluded scope would be destroyed.

This didn't work for transclude contents that didn't contain any elements - most
importantly in the case of the transclude content containing an element transclude
directive at its root, since the compiler swaps out this element for a comment
before a destroy handler could be attached.

BREAKING CHANGE:

`$transclude` functions no longer attach `$destroy` event handlers to the
transcluded content, and so the associated transclude scope will not automatically
be destroyed if you remove a transcluded element from the DOM using direct DOM
manipulation such as the jquery `remove()` method.

If you want to explicitly remove DOM elements inside your directive that have
been compiled, and so potentially contain child (and transcluded) scopes, then
it is your responsibility to get hold of the scope and destroy it at the same time.

The suggested approach is to create a new child scope of your own around any DOM
elements that you wish to manipulate in this way and destroy those scopes if you
remove their contents - any child scopes will then be destroyed and cleaned up
automatically.

Note that all the built-in directives that manipulate the DOM (ngIf, ngRepeat,
ngSwitch, etc) already follow this best practice, so if you only use these for
manipulating the DOM then you do not have to worry about this change.

Closes #9095
Closes #9281