Add "overflow: auto" to all comment boxes

Summary: Set these all to "overflow: auto".

Test Plan:
Made comments like "MMMMMMM..." in:

• Differential comment preview.
• Differential comment (saved).
• Maniphest comment preview.
• Maniphest comment (saved).
• Differential inline comment draft.
• Differential inline comment preview.
• Differential inline comment (saved).

Also tested code blocks.

Reviewers: nh, btrahan, jungejason

Reviewed By: jungejason

CC: aran, epriestley

Maniphest Tasks: T883

Differential Revision: https://secure.phabricator.com/D1641