Updates to URL Field layout and behaviour of directive version

- HTML template refactored to use "help-block" class to show message below input field as well as now showing the checkbox opt in/out below the input field
- Change in behaviour to only show checkbox if url entered is invalid and/or checkbox has previously been unchecked
- Directive updated to use "forceValid" variable to implement new behaviour for template
- e2e tests updated to be referencing "help-block" class instead of "text-danger"
Merge pull request #126 from stulees/chore/url-field-updates

Updates to URL Field layout and behaviour of directive version