Add the show_controls parameter to build_diff_fragments which

determines if expansion controls should be shown for the diff

fragments. This parameter defaults to False.
The comment_diff_fragments view sets this show_controls parameter

to True if and only if it is showing the fragments in a review and

not the edit review dialog.

Tested on local server:

Review comments contain expansion links.
Edit review dialog does not contain expansion links.
Emails do not contain expansion links.