Comments: Form is missing label element

Raised by @davidakennedy:

> The comment form area is missing a label element. The hidden textarea that expands the form appears to try and compensate for this, but it’s still not as intuitive for a screen reader user.

More information on the use of the label element can be [found here](https://developer.mozilla.org/en/docs/Web/HTML/Element/label).

Locate the comment form and its hidden textarea in the Jetpack codebase. Inspect the rendered form with accessibility tools or a screen reader, then verify that the comment control has an associated label and remains usable when the form expands.

Written by the indexing model from the issue text.