Re: mbox download username/password

Further to discussion at PGCon this morning, webkit based browsers no
longer display the realm text when displaying password prompts. This
was used by our archives code to tell the user what username/password
to enter when downloading mbox files (which are loosely protected
against bot downloads as they contain email addresses).

The attached patch adds a note to the appropriate template to convey
the username/password info on the page. It's currently untested as I
don't have a pgarchives test environment.

Magnus - can you test/apply please?