drop xhtml content-type support (closes #2065651)
* HTMLStream does not care about xml any more
* reqquest.demote_to_html and .xhtml_browser are deprecated
* web config: drop force-html-content-type option
* adjust tests
from cubicweb.schemas.base import in_group, CWSourceSchemaConfig
# copy __permissions__ to avoid modifying a shared dictionary
in_group.__permissions__ = in_group.__permissions__.copy()
in_group.__permissions__['read'] = ('managers',)
cw_for_source = CWSourceSchemaConfig.get_relation('cw_for_source')
cw_for_source.__permissions__ = {'read': ('managers', 'users'),
'add': ('managers',),
'delete': ('managers',)}