Comments: Keyboard accessibility

Raised by @davidakennedy:

> The tab order is off in the form. I can’t get to the email opt-in check boxes. It also skips over the social login links. But you can find them if you tab in reverse.
#### Steps to reproduce the issue
- Open any post/page on which Jetpack's comment form is enabled.
- Tab to the comment form using only your keyboard and enter a comment into the main form.
- Next, select the tab button again.
#### What I expected

After entering my comment and tabbing, I expected to be taken to the next option provided by the form: The checkbox next to **Notify me of new comments via email**.
#### What happened instead

I was taken straight to the form's submit button. The checkbox options were skipped over.


Start at the Jetpack comment form on any post or page with comments enabled and reproduce the keyboard-only tab sequence described in the issue. Trace the form's focus order, then verify that the email opt-in checkboxes and social login links are reached in forward order before the submit button.

Written by the indexing model from the issue text.