[WIP] First pass on form formatting

Also brings the input height inline with the designs, down from 36px to
30px and removes duplicate box-sizing properties.
Formatting for forms

http://0.0.0.0:5000/en/styleguide/forms (Examples on
styleguide-form-examples branch)
- Add .form__item and .form__group structural components
- Add %form-input placeholder
- Add inset label component style .form__input-label
Formatting for forms [continued]
Scope .is-errored to .form__item and rename _forms to _form

Have left the .is-errored selector there as well, as this is being
referenced in other projects. We could consider moving this to a helper?
Update .form__ component naming as per team discussion
Add $input-button-height variable

Will be used to match an upcoming component, .button—input’s height to
that of the input.
Merge branch 'component/form-item' of github.com:moneyadviceservice/dough into component/form-item

Conflicts:
	stylesheets/base/_forms.scss
	stylesheets/components/common/_form.scss
	stylesheets/lib/_variables.scss