Customizer: make utilities.less inclusion mandatory

Fixes #13305.

Remove outdated rtl nav links

move Grunt info from README into HTML docs

Fixes #13470

emphasize that bootstrap-rtl.css & bootstrap.css should not be used together

Fix .visible-print deprecation version #.

unlock & bump grunt-jscs-checker to ~0.4.3

Allow for resetText of a button to be a falsey value; fixes #13466

fires closed.bs.alert *after* DOM detach() as per #12379

bump grunt-jscs-checker to v0.4.1

Per #13512 & https://github.com/mdevils/node-jscs/issues/357

import rules from Ratchet's JSCS config with which we already comply

Improve accessibility of close buttons in modals & alerts

Officially support Android Firefox

Adds textual caption to the close buttons. The "X" multiplication sign isn't very meaningful to screenreaders.

Credit: https://github.com/paypal/bootstrap-accessibility-plugin
Per:
    https://github.com/paypal/bootstrap-accessibility-plugin/blob/master/README.md#alert
    https://github.com/paypal/bootstrap-accessibility-plugin/blob/master/README.md#modal-dialog

Fixed navbar radius of example at docs

Remove sr-only from components/dropdowns example

possibly less confusing option, as per discussion, is to always show the
dropdown trigger button (both live and in that code example)