Added sandboxing to CommentDetailDisplayHook

This change adds exception handling so that broken extensions which use
CommentDetailDisplayHook won't take down the reviews page. This includes new
unit tests for this behavior.

Testing Done:
Unit testing - tested both render_review_comment_detail and
render_email_comment_detail with and without sandboxing

Reviewed at https://reviews.reviewboard.org/r/5709/
Add some error handling to autocomplete.

This is a minimal version of how the backbone models handle errors, except that
I put the JSON parsing into the error callback rather than directly into the
autocomplete library, in order to keep the latter more generic.

Testing Done:
All unit tests pass, except some VCS-specific ones, which were skipped.

Reviewed at https://reviews.reviewboard.org/r/5766/
Merge branch 'release-2.0.x'