Show buttons for all review request field editors.

Users were getting confused about how to save fields on the review
request page, since we removed the buttons in Review Board 2.0. This
change helps to solve that by adding back the buttons.

The fields on the side of the review request now have buttons once
again. Instead of taking up precious space on the field's line, they now
appear below the field instead.

The buttons are a bit smaller to help visually fit, and spacing is added
to help keep them from bumping up against neighboring fields. Any
fields below will simply shift down while the buttons are shown.

Fixes bug #3474
Reviewed at https://reviews.reviewboard.org/r/6179/