Add a site setting for the static URL
Add a site setting for the static URL

This patch adds a field to the general site settings form for
configuring the URL of served static media.

Testing Done:
Updated the field and verified requests were made to the new URL.

Reviewed at https://reviews.reviewboard.org/r/7979/

sgallagher/reviewboard,sgallagher/reviewboard,davidt/reviewboard,reviewboard/reviewboard,brennie/reviewboard,reviewboard/reviewboard,davidt/reviewboard,chipx86/reviewboard,chipx86/reviewboard,davidt/reviewboard,sgallagher/reviewboard,davidt/reviewboard,chipx86/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,brennie/reviewboard,reviewboard/reviewboard,sgallagher/reviewboard,brennie/reviewboard,brennie/reviewboard