Fix extensions' config/ and db/ admim links with custom SITE_ROOTs.

In the case of custom SITE_ROOTs, the calculation of the extensions'
'db/' and 'config/' seem to use the reverse function for extension_list
method and then append it to the DynamicUrlResolvers' add_patterns
method. This adds an additional SITE_ROOT pattern as
reverse(extension_list) itself returns the path with the SITE_ROOT. So,
instead of the url for SITE_ROOT='/reviews/' being

reviews/admin/extensions/rbcustomreports.extension.CustomReportsExtension/config/
it turns out to be
reviews/reviews/admin/extensions/rbcustomreports.extension.CustomReportsExtension/config/

Fixed this by lstrip-ing the SITE_ROOT after getting a reverse function
so that dynamic url resolver adds it properly. This should work for
SITE_ROOT='/' or custom SITE_ROOTs.

Tested with SITE_ROOT='/' and SITE_ROOT='/reviews/', installed an
extension and testing the "Configure" link.

Reviewed at http://reviews.reviewboard.org/r/3874/