The Markdown reference links were just a bit too high up, which was

more noticeable now that we have a checkbox. This fixes them to be

better aligned.
Plain text fields were also getting a minimum height when they shouldn't

have, due to the way we checked for the presence of a minHeight option.

This resulted in the minHeight being set to the default of 70 when 0 was

passed. We now check for the existence by comparing to undefined.

Checked the Markdown Reference links on all fields and in the comment

dialog. Saw that it was properly positioned everywhere.
Opened a plain text field with only one line of text and saw that it

had the proper height, matching what the Markdown equivalent had.