Don't redefine gCommentIssueManager.

This was causing a page load error. gCommentIssueManager was being set
in the page template, but being overridden locally in reviews.js.
Fix the Open Issue checkbox.

The default value for the Open Issue checkbox after loading a comment
was based on this.defaults.openIssue, but this.defaults was a function.
It now calls that and access openIssue from the resulting object.