Add an extra newline after <textarea> to preserve textarea content beginning with newlines

Summary:
Fixes T8707. See that task for discussion. Browser behavior is apparently to ignore a newline immediately following a <textarea>, and ostensibly has been since the early 1800s.

This is the same fix Rails used when it encountered this issue in 2011, which gives me some confidence it is correct.

Test Plan:

• Edited a task with leading newlines in the description.
• Newlines were preserved correctly across multiple edits.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8707

Differential Revision: https://secure.phabricator.com/D14688