Fix the "Open an issue" checkbox.

There were a handful of problems with the "open an issue" checkbox in the
comment dialog, mostly introduced when I marked the javascript strings for
translation:

- The form field used "name" instead of "id", which meant that clicking on
  the label didn't work.
- The "Open an issue" text has an underline on the "i", presumably to denote
  an access key, but it was formatted into the template with HTML escaping.
- The <label> element has an accesskey attribute that we weren't defining.

Testing done:
- Created new comments and edited old ones, changing the issue state.
- Verified that clicking on the label changed the state of the checkbox.

Reviewed at https://reviews.reviewboard.org/r/4465/