Expand comments on page load only when the anchor points into them

Summary:
Ref T3099. Currently, we expand comments in Differential when //any// anchor is present. This creates a scrolling issue described in T3099. Instead, expand them only when the anchor contains `comment`.

We can go further here, but this should fix the immediate issue.

Test Plan: Viewed `/D22`, `/D22#toc`, and `/D22#comment-3`. The first two did not expand comments; the last one did.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3099

Differential Revision: https://secure.phabricator.com/D5836