Define BASE_URL and DATA_URL js variables in the login template. Closes #13385185
Those are expected to be defined in almost every js files. They were not before
this patch because login form doesn't use the regular main template.
usr/lib/python2*/*-packages/cubicweb/web
usr/lib/python2*/*-packages/cubicweb/wsgi
usr/share/cubicweb/cubes/shared/data
usr/share/cubicweb/cubes/shared/wdoc