controllers: don't pass rendered templates in context variables

Some controllers used the followifng pattern:
- render a data template into a context variable
- for partial (ajax) requests, return the contents of this variable
- for full-page requests, render the full page, which expands the value of
the context variable

Instead, avoid context variables let the controller simply render the full or partial page, and let
the full page template include the partial page.

Remove this context variable for templating and use render exclusively.
From templates, use %include instead of context variables.

This in line with the suggestions in the Pylons documentation:
http://pylons-webframework.readthedocs.org/en/latest/helpers.html#partial-updates-with-ajax