Accessibility updates - 28th August

Just a quick update to let the group know what we've deployed this week!

So far, we've:
- tied labels to inputs on all forms (using aria-labelledby)
- removed the extra 'main' region on the Group page
- applied 'aria-hidden' to the 'Times' symbol used to indicate a 'Cancel' button
- done a comprehensive sweep of the Thread page, activity items in particular should sound much better on screen readers
- converted stars to checkboxes

We'll add some activity items to the testing accessibility thread