Added a new hook to add new element rules to the Whitelister.
Add unit tests for DbWhitelister, including ability to override the whitelist via hooks
Move construct_whitelister_element_rules hook logic into DbWhitelister, as the whitelist module should be Wagtail-agnostic
add documentation for the construct_whitelister_element_rules hook
add changelog entry for construct_whitelister_element_rules hook
Merge branch 'kcl-ddh-master'