Remove Bootstrap classes from base.php and create equivalent LESS

I removed the `container` and `row` Bootstrap classes from `base.php` and use the Bootstrap LESS mixins instead.

I realize this is a good step in making Roots framework agnostic but my main intent is to facilitate custom layouts without the need to create a custom base file every time.

For example, a page requiring a full width content area can easily utilize the `:not()` selector on `.wrap` based on body classes.
What about using `.container-fixed()` mixin instead of extending `.container`? There's not necessarily a functional benefit, but it is consistent with the make-row and make-*-column mixins that you've been using.