Relax slug validation to not fail on cosmetic issues 

ActiveSupport's [parameterize](http://apidock.com/rails/ActiveSupport/Inflector/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).

Relaxing this in the base class has allowed removing a lot of the special-case behaviour from the whitehall formats.