Use input helper classes on common attributes

* Avoid using divs as grid wrappers to restrict width of form elements.
The admin gem provides classes for this.
* Remove beta help text, the explanation is extraneous now the feature
has been introduced and understood
Use input helper classes on business support form

* Constrain widths using classes rather than grid wrappers
Whitespace: alignment of tags in business support
Use input helper classes on campaign form
Use input helper classes on answer form
Use input helpers on remaining forms

* Tidy up whitespace for input attributes
* Use more rows for introductory paragraphs (2 isn’t enough for most
content, scrolling the small boxes is painful)
Merge pull request #283 from alphagov/input-class-helpers

Replace grid wrappers with input helper classes