Fix tab order of comment form elements in river view.

The original bug was that, when having more than one comment form on the page,
pressing Tab from one comment input box would not go to the Comment button
related to that input field, but rather would jump to the next comment form's
input box.
Merge pull request #30 from Libertree/fix-comment-form-tab-order

Fix tab order of comment form elements in river view.