#966768 by tekante - Undefined property: stdClass:: in _block_get_renderable_array when saving a configured context

#863298 by donquixote, yhahn: Only reset the block info cache when a form is submitted in the admin UI.

Introduce hook_context_node_condition_alter() to allow other modules to easily add their plugin to node-related events.

Splice context_preprocess_page() into preprocess stack immediately after template_preprocess_page().