chore(jshint): enforce jshint for tests

Closes #7264
docs(ngCookies): use GFM code-snippet rather than example tag

The example tag creates a big ugly white rectangle on the docs page, and this is not very helpful
and kind of looks bad. So GFM snippets are a better way to go.

This fix also removes the unnecessary example heading from the $cookieStore page, as there has not
been an example use of $cookieStore for 2 years now.

Closes #7279
feat(misc core): add ngId directive

Add a way of dynamically updating the id attribute, using either a string or an object.
Save the old id automatically in case the string or the object evaluates to falsy.