Make it possible to have per-language site data.

This is #851.

The list of variables is currently limited to a subset by the
TRANSLATABLE_SETTINGS value in nikola.py:

    TRANSLATABLE_SETTINGS = ('BLOG_AUTHOR',
                             'BLOG_TITLE',
                             'BLOG_DESCRIPTION',
                             'LICENSE',
                             'CONTENT_FOOTER',
                             'SOCIAL_BUTTONS_CODE',
                             'SEARCH_FORM',
                             'BODY_END',
                             'EXTRA_HEAD_DATA',)

Moreover, some things (like CONTENT_FOOTER) are error-prone
and require big changes to conf.py.in.