Fix scrolling issues in the comment dialog in Chrome and Firefox.
Fix scrolling issues in the comment dialog in Chrome and Firefox.

We've started seeing issues recently with the comment dialog in both
Chrome and Firefox when attempting to scroll the text field. On Chrome,
any attempt at scrolling the text area would also result in the page
being scrolled. On Firefox, scrolling might not even work at all.

This was in part caused by some old `overflow` style rules we had for
the scroll container, which at some point was necessary to fix some
styling issues, but these days interacts poorly with modern scrolling
behavior.

Removing these rules and adding `overscroll-behavior-y: none` fixes
this. This new property tells browsers not to continue scrolling when an
inner scrolling element is scrolled to the very top or very bottom. It
also seems to correct some quirky scrolling issue currently specific to
Chrome, which was causing both the page and CodeMirror to scroll
simultaneously.

Testing Done:
Tested scrolling in the comment dialog and text fields in Chrome, Firefox,
and Safari.

My version of Safari is a bit old, and does not support `overscroll-behavior`,
but scrolling still works fine until the boundary of the scroll range is hit.

Reviewed at https://reviews.reviewboard.org/r/11661/

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