Refactor: make lists more accessible to screen readers

[bmakuh]

• Bullet and ordered list items have aria-label attributes attached to simulate proper semantic HTML
• Fake .ql-ui decorations have aria-hidden="true" applied so that they are not read aloud as "image"
• Checklists have role="checkbox" aria-checked="true|false" applied

This is an improvement, but it is not bulletproof. There are certain text transformations where the aria-label can get out of sync with the text.