R1-213: Template code audit

[patrickcuagan]

Ticket: https://torchbox.atlassian.net/browse/R1-213

This PR does a template audit of breakages that could happen if a referenced page is deleted. What we checked are:

• Usage of page.<attribute> e.g. page.specific, page.title, etc.
• Usage of {% pageurl <page> %}.

We tested by:

• In any page type that has a page chooser, reference a page to a field that has a page chooser.
• Delete that page.
• View the page to verify that the page can still be viewed.

[neilbulloch]

Hey @patrickcuagan this looks good to me code wise. It's hard to understand the full impact throughout the codebase, but the changes look sensible to me.