Remove obsolete scenarios from default test suite

Ignore scenarios tagged with `@replacement_available` when
executing `rake features` or `rake features_html`.

Allow scenarios to be progressively excluded as the corresponding
unit tests become available.
Add development test suite to default rake tasks

These specs document interfaces that are not public; making
them visible, however, should make the test suite refactoring
easier to follow.
Refactor extract method `integrate_with_devise_trackable!`

Add Devise trackable integration specs
Disable outperformed Cucumber scenarios...

... related to the Devise trackable integration

The unit tests which cover the integration with the Devise trackable
strategy are sufficient to prevent any regression while the Devise
trackable interface does not vary.

The (slow) Cucumber feature remains available and should be used
when updating the Devise dependency.
Disable outperformed Cucumber scenarios...

... related to the token authentication handler behaviour activation.

The new 'token authentication handler' interface (and the corresponding
module) makes straightforward to unit test that the controllers do not
require authentication (behave as token auth. handlers) before asked to
do so.
Disable outperformed Cucumber scenarios...

... related to the `sign_in_token` option

The specs tagged with `sign_in_token_option` match almost exactly
the original Cucumber feature.

Use: `rspec --tag sign_in_token_option` to run them in isolation.
Disable outperformed Cucumber scenarios...

... related to the `header_names` option