Improve FormElement Accessibility

[alexpaxton]

Closes #494

This change should allow inputs to be rendered inside labels (which has better accessibility) without breaking the API

Changes

• Render FormLabel as a <div /> instead of <label /> unless htmlFor is passed in
• Add optional htmlFor prop to FormElement & FormValidationElement
  • If a value is passed in FormElement renders as a <label /> instead of <div />
• Bump label font size from 12px to 13px

Screenshots

Checklist

Check all that apply

• Updated documentation to reflect changes
• Added entry to top of Changelog with link to PR (not issue)
• Tests pass
• Peer reviewed and approved
• Signed CLA (if not already signed)