diverse fixes for project url

 * Force leading and trailing slashes in base_project/kernel_url
 * Add support for the prefix in template for static files
 * Fix some forgotten quotes
 * remove old make_static_url which is now a jinja2 macro

fixes-2720
Merge pull request #2723 from Carreau/fixes-2720

diverse fixes for project url

- Force leading and trailing slashes in base_project/kernel_url
- Add support for the prefix in template for static files
- Fix some forgotten quotes
- remove old make_static_url which is now a jinja2 macro