A11y: add role="button" to span subIndicator

[jenlampton]

This is an alternative to #204 since that approach seems quite invasive.

Adding a role attribute could have the same benefit of using a <button> element, and would improve the current state of things significantly without requiring a rewrite.

We do still need to find a way to move the span outside the anchor tag, but I'll leave that for a follow-up.

[laryn]

I'm noticing that when using the split menu functionality, the +/- button is not keyboard accessible. We could add tabindex="0" to fix that, right? Is that similar to what this ticket is trying to achieve?