Fixed #11919 -- Added a DPASTE_URL setting to customize the dpaste featu...

...re.

https://code.djangoproject.com/ticket/11919
maybe DEBUG_DPASTE_URL? (I guess the feature still won't work with arbitrary pastebins because of the particular set of post data names

Also add it to the relevant headings in the "topical settings guide" on this page, and a mention in the release notes.