Fix preview width and add label

* Make the preview area the same width as the editing area
* Add a "Preview" label above the preview area. Right now it
  just looks like the other labels on the form; we might want
  to style it a little differently but I'll leave that to
  @juliaelman.

This addresses part of #15.
Merge pull request #21 from caktus/feature/preview-width-and-label

Fix preview width and add label