Brings back Chapter/Section note deletion

Removal button was removed from index pages and moved to edit pages instead
Remove APP_SLUG, admin app does not use it
Fix menu item highlighting

Related to removal of APP_SLUG
Remove several redundant views

Related to splitting out notes and synonyms into different namespaces
Merge pull request #12 from alphagov/59576426-enable-note-deletion

59576426 enable note deletion