studio - revises z-index/element depth levels for modals, notifications, prompts, and alerts to use universal .depth Sass extends

This reverts commit e13556b8b6b54b2791fe7bb1ba2ae5cde10114ce. This is because the
JS files need to be loaded in a specific order for the tests to run properly, and
we don't yet have a way to declare that.

removed symbolset, replaced with fontawesome for gradable status; added transform-origin for saving notification animation to remove wobble

studio - changes all specific ss-icon-based selectors to use Font Awesome/generic icon- class name in notifications/prompts/alerts Sass

removed symbolset, replaced with fontawesome for alerts, notifications, and all references in js files

Also, a SystemFeedback model no longer has a `type`, it has an `intent`. This is
because SystemFeedback views have `type`, as well, and this prevents a naming
collision.

To reduce redundancy in tests

Doing so overrides the AJAX error popup

studio - removes the .is-fleeting class/animation that is handled by backbone's min/max time for notifications

studio - revises copy of server error notification to include a default message and removes associated actions from notifications

Model shouldn't know about things like close buttons, visibility, and so on