mixer: fix requirements

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
legacy: new webinterfaces registry

* Adds new registry for legacy webinterfaces.  (closes #2239)

* NOTE Adds PendingDeprecationWarning for *ALL* legacy pages.

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>