DAR-2114 New styleguide component: modals

* added new componented to UI Styleguide -- modals,
* added possibility to display a styleguide component and bind into it event which shows another styleguide component's example,
* made template exception's message more descriptive,
* back-ported from dev fix for loader.js flooding,
* created blackout mixin.

More info: [DAR-2114](https://wikia-inc.atlassian.net/browse/DAR-2114)