[IMP] base, website: correctly use inheritance when creating qweb rendering context

Base render method now calls a prepare_qcontext method used to prepare
the qweb context used for the rendering. Website override of ir_ui_view
now simply inherits this method to add its own set of values instead of
doing strange things with the base render override.

Qweb context should be the same after this commit, i.e. containing a lot
of duplicated things like a copy of the context. The only change is that
user_id and res_company are now directly supported in base rendering
instead of only in website version of rendering. Future commits will
use those values.