[request] gather all base_url logic in a single place (closes #2200756) * Handle ``https base-url`` in the core code: ``./req.py`` and ``web/request.py`` * Remove custom ``base_url`` handling if web request

bgcolor = '#000000'
stylesheets = ['%s/cubicweb.css' % datadir_url]
logo = '%s/logo.png' % datadir_url
lazy = lazystr('%(bgcolor)s')