Since using a browser is only for selenium it doesn't really make sense
to have a separate class for handling it there. This brings a lot of the
if/else out of the main SystemTestCase class and into the Driver class
so we can abstract away all that extra work.

`Capybara::RackTest::Driver` does not support taking screenshots. If call
`#save_screenshot` on `Capybara::RackTest::Driver` will raise the error.

```ruby
Error:
UsersTest#test_visiting_the_index:
Capybara::NotSupportedByDriverError: Capybara::Driver::Base#save_screenshot
```

To prevent errors, if driver does not support screenshot, do not call it.

Previously, `driven_by` would change the Capybara configuration when
the test case is loaded, and having multiple test classes with different
`driven_by` configs would fail as the last loaded would be effective.

* Override integration test default host

Integration tests automatically set the default host to
'http://example.com'. This works fine for integration tests because they
are not real browser sessions, but doesn't work fine for system tests
because they are real browser sessions.

We can override this by setting the `host!` in `before_setup. The
`Capybara.always_include_port` will allow the test to look at
`127.0.0.1:port capybara picks` and properly redirect the test.

Any application can override this by setting the `host!` in
their system test helper. Generally though, applications are going to be
using localhost.

In this commit I also moved the setup and teardown into their own module
for tidiness.

* Move teardown settings into system test case

These configuration options can be put into the system test case file
instead of the generated system tests helper file. This is an
implementation detail and therefore shouldn't be generated with the
template.

We only want the file name to include the word `failures` if it failed,
not any time the user wants to take a screenshot during a test run.

* Move system tests back into Action Pack
* Rename `ActionSystemTest` to `ActionDispatch::SystemTestCase`
* Remove private base module and only make file for public
`SystemTestCase` class, name private module `SystemTesting`
* Rename `ActionSystemTestCase` to `ApplicationSystemTestCase`
* Update corresponding documentation and guides
* Delete old `ActionSystemTest` files