On a page of search results, the "Edit on GitHub/Bitbucket" link tries
to send the reader to a file "search.rst" or "search.txt" in the
repository. Since the search results only exist as a template, this is
a 404'd link.
This change to the template means that the "Edit on GitHub/Bitbucket"
link is not shown on search pages, getting rid of this broken link.
Currently only source code hosted on GitHub or BitBucket are supported
to be linked. This PR adds the ability to link to an arbitrary source
code host URL. To use it, html_show_sourcelink must be True, and
source_url_prefix must be assigned to be the URL prefix.
