Add isLegend prop to FormLabel

[dg-harris]

Github Issue or Trello Card

This PR addresses this issue:
#847

What type of change is this?

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

Completeness Checklist

• TESTS: My changes maintain the baseline required test coverage, as specified by code climate analysis.
• DOCS: All new component work is covered in that component's Overview docs.
• DOCS: All new component work is covered in a component Playground.
• My code has no linting or typescript compile warnings.
• My work is tied to a Github issue and satisfies the acceptance criteria (if applicable) of the corresponding issue.

Quality Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codeclimate]

The SHA range provided is invalid

The test coverage on the diff in this pull request is 100.0% (95% is the threshold).

This pull request will bring the total coverage in the repository to 97.9%.

View more on Code Climate.