Add more accessibility labels for screen readers

Summary:
Depends on D19594. See PHI823. Ref T13164.

• Add a label for the "X" button in comment areas, like "Remove Action: Change Subscribers".
• Add a label for the floating header display options menu in Differential.
• Add role="button" to PHUIButtonView objects that we render with an <a ...> tag.

Test Plan:
Viewed a revision with ?__aural__=true:

• Saw "Remove Action: ..." label.
• Saw "Display Options" label.
• Used inspector to verify that some <a class="button" ...> now have <a class="button" role="button" ...>. This isn't exhaustive, but at least improves things. A specific example is the "edit", "reply", etc., actions on inline comments.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13164

Differential Revision: https://secure.phabricator.com/D19595