Overhauled the way the entity template retrieval

Overhauled the way the entity template retrieval

- The way of retrieving the right entity template is significantly
simplified (and vastly improved) and now get_entity_template() is
used as well as is_entity_landing_page(). As a result straggler
pages (without a designated entity parent) are easily given the
'default' template.
- The sidebar has been fixed and should show up (or not) as
expected, on no-sidebar templates and GLM Associate required pages
alike.
- The navigation lib file has been fixed, allowing for proper
sidemenu items even on pages without an entity parent.