Show diff/interdiff links from rbt post output

This change makes it so 'rbt post' will print both the URL to the new diff as
well as the URL to the review request.

Testing Done:
I have tested this multiple times on my command line, when posting review
request via rbt. I also attached pic about the test case.

Fixes bug 2929.

Reviewed at https://reviews.reviewboard.org/r/5434/