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.

# -*- coding: utf-8 -*-
# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
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%(long-license)s
"""%(distname)s specific hooks and operations"""