[datafeed xml parser] fix typo

[twisted] move out url logic from twisted (closes #2200593) Data, fck and static file are now served by cubicweb controller by default. Some hackish url rewriting is necessary to have them selected. This is necessary because cubicweb does not selection controller on multi level path. - 'Expires' header seems to conflict with Cache-Control set by HTTPCacheManger (don't know yet why HTTPCacheManager is involved here) mod_wsgi is now usable in simple cases (e.g. still requires persisten sessions).

[cwconfig] improve assert error message. Single tuple element are processed in a non trivial way.