Add Modal.js + Add global z-index scale

[mcaskill]

Add accessible modal dialog component and ordered z-index map to standardize the layering of our components.

Dialog Features:

• Dialog supports being shown on page load via data-modal-autoshow attribute.
• Dialog supports being shown at most once via the data-modal-show-once attribute.
• Dialog persists dismissal via localStorage, by default. This can be switched to sessionStoage via data-modal-show-once="session" attribute value.
• When Load handles "loading" event, executes Modal.hide on all shown modals.

Required:

• NPM dependency a11y-dialog v7.3.0

Added:

• JS class Modal that uses and enhances a11y-dialog.
• CSS component .c-dialog via partial '_modal.scss' with basic styles (adapted from vendor's example).
• Basic HTML example of data-module-modal.
• Default Sass z-index scale $overlay-zindexes: dropdown, sticky, fixed, offcanvas, modal, popover, tooltip, transition.
• Sass function overlay-zindex() to fetch the z-index for the corresponding element (based on OZMap).

Note:

• I'm open to suggestions to rename overlay-zindex() function and map. Maybe global-zindex is a better term?

[devenini]

Overall I like the idea of building re-usable components. I'm just not sure if the boilerplate is the best place to put these tools. I think a "playground/demo type" repo with all the components would be best. Since this repo is our project starter and that means we will have to clean/remove all unused components.

[mcaskill]

Closing in favour of moving this component to locomotivemtl/cookbook#7.