Update kbn-accessible-click directive to differentiate between links and buttons.

[cjcenizal]

Per #11513, only elements which are meant to act as buttons should have the button role and respond to the space bar keypress event. We need to update the kbn-accessible-click directive to allow engineers to specify whether it should behave as a button or not:

1. If the markup is kbn-accessible-click="button", the element should act as a button.
2. If the markup is kbn-accessible-click, the element should not act as a button.

We'll have to update each instance in which this directive is used, to specify the correct role.

See https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/ARIA_Techniques/Using_the_button_role for more info.

[cjcenizal]

After talking about this with @tsullivan we agreed that it doesn't make sense to use this directive on an element which is acting as a link. In that case, you'd just use an anchor tag with an href. It only makes sense to use this directive on an element which acts as a button or otherwise responds to the SPACE keypress, like checkboxes and radio buttons. See http://webaim.org/techniques/keyboard/ for more info on these types of elements.