[Design] Update UI for Form Structure components

[nichole-aquino]

Overview

Form builders can easily add/edit Form Structure components (question set, repeating question set)

Context

Reference:

• Eng ticket: [Eng] Update UI for Form Structure components #458
• Figma mockups
• USWDS components: text input and text-area, unstyled button

Rename components:

• Fieldset: Change to Question set
• Repeater: Change to Repeating question set

On the build screen, for the heading:

• Relabel the input field to ‘Question set label’
• For the repeating question set, add option to edit the button text
• Add optional help text component; the field grows as needed (GH ticket #373)
• Add optional information box component 'Add supplementary information or instructions'(GH ticket #415)

On the build screen, for the ‘Add question to fieldset’ link

• Replace grey link with unstyled button
• Replace copy to ‘Add question to set’

Acceptance Criteria

The changes should be made to the following question types:

• Question set (aka field set)

• Repeating question set (aka repeater)