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.
Fix cancel action on deletion confirmation form
Currently crashed with RequestError: invalid action '__action_cancel'.
It's broken as of https://hg.logilab.org/master/cubicweb/rev/cb217b2b3
This fix is similar as the one done in
https://hg.logilab.org/master/cubicweb/rev/9f1c89e7426d
Closes #13521493