Remove dash from 'assurance' fields

The dashes prevent the controllers from directly binding to method arguments so we've taken them out.
Remove description field from listing

We are no longer displaying the description on the SSP home page, so no need to have it as a separate field of the model (it is still available from the saved page response).
Add trailing slash to URL

The URL was wrong - needs a trailing slash.
Add validation utils

Helper class for static methods to do various form data validations.
Update validation in question page controllers

This updates validation in controllers to be up to date with the current content version as far as possible.
Merge pull request #63 from alphagov/more_validation

More validation