Add some more tests for slug validations
Relax slug validation to not fail on cosmetic issues

ActiveSupport's parameterize method replaces characters using a regex,
but then also does some cosmetic tidying (eg squashing consecutive
dashes).  This has led to false positives from apps that use gems like
friendly_id which use --n to de-dup slugs (eg Whitehall).
Clean up now that default slug validator is better

A lot of the Whitehall-specific behaviour was to deal with consecutive
dashes.  This is now handled by the base class.

DetailedGuides always live at the root level, so no longer need any
special case handling.