mod_base: minor: tidy up in controller_postback.
mod_base: add template action.

This makes it easier to send template data as response to postback and submit events.

Without template action::

   {Output, Context1} = z_template:render_to_iolist("response.tpl", z_context:get_all(Context), Context),
   z_context:add_script_page(Output, Context),
   Context.

With template action::

   z_render:wire({template, [{template, "_license_data.tpl"}|z_context:get_all(Context)]}, Context).

It also avoids a round-trip to the page session, by keeping the information in the context at all times.