Hotfix/#460 slick carousel a11y fixes

[Cheffheid]

Doesn't necessarily close a ticket. It does improve the situation for #460.

DESCRIPTION

I've added some additional features to help SlickJS do better at accessible content:

• A pause button is added to the carousel through JavaScript after it initializes.
• Made use of wp.a11y.speak() to update an aria-live region with the carousel status, specifically when the pause button is used or when the arrow navigation is used.

The dot navigation didn't need any additions, since it seems to be using a tablist role to provide feedback about the amount of slides there are and what button is activated (eg. "3 of 3").

SCREENSHOTS

No significant styling was added. The pause button will sit in the bottom left, below the slider, by default.

I do have a screenshot of a test carousel and the NVDA speech viewer:

OTHER

• Is this issue accessible? (Section 508/WCAG 2.0AA)
• Does this issue pass all the linting? (PHPCS, ESLint, SassLint)
• Does this pass CBT?

STEPS TO VERIFY

Create a page with a carousel on it and verify that the pause button appears. If you have NVDA or some other screenreader installed, you should be able to use that to test the verbal feedback by clicking/keyboarding the Previous/Next and Pause buttons.

[coreymcollins]

I'm going to put some time into testing some other options to see how accessible they are for our uses, as well as how easy they are to customize to our various needs.

A few I'm looking at:

• https://nickpiscitelli.github.io/Glider.js/
• https://pawelgrzybek.github.io/siema/
• http://ganlanyuan.github.io/tiny-slider/demo/
• http://idangero.us/swiper/demos/ (Swiper seems interesting but also seems like a lot)

I'm also in the camp of just not using carousels ever, but I know that's not a reality we live in when building sites for others. The main thing I like about the 3 options above (and I'm sure there are more) is that they're actually maintained, whereas Slick hasn't had an update since last April with 930 outstanding issues and little to no feedback on those issues.

Tiny Slider seems nice as it's the only one of the group above that includes an option for a pause button (that I can find, at least) and mentions accessibility in the Github repo.