Un-escape markdown text for 'rbt patch -c'
The patching step was including escaped characters in the description and

testing done, which was annoying to look at, and made it so in the case where

the first line of the description was the same as the summary except for

escaping, we wouldn't collapse them together.
This change adds some code to do the same as markdown_unscape() from

reviewboard.reviews.markdown_utils.

Ran rbt patch -c for a few changes on reviews.reviewboard.org that had

escaped descriptions and testing done. Saw the expected results.