Fix a11y violations reported in ContextualMenu examples

[KevinTCoughlin]

Pull request checklist

• Addresses an existing issue: Fixes ContextualMenu A11y Pass #6360
• Include a change request file using $ npm run change

Description of changes

This PR addresses the a11y violations for ContextualMenu examples. There is no impact to the lib, but they were flagged in #6360 so we opted to address them so folks don't copy invalid code.

Fixes:

• Bug 1 : [Accessibility][Forms and validation]: 'Actions' label is not associated with its corresponding control.
• Bug 2 : [Accessibility][Screen reader]: Label or place holder is not provided for the Edit field(250) present under Contextual Menu with Sub Menus
• Bug 12 : [Accessibility][Programmatic access]: id attribute value must be unique.

ContextualMenu component a11y fixes to follow in subsequent PR.

Focus areas to test

1. Load the ContextualMenu example pages.
2. Verify that id attributes are unique.
3. Verify that inputs have labels or aria-label defined.

Microsoft Reviewers: Open in CodeFlow

[natalieethell]

Looks like snapshots just need to be updated.

[msft-github-bot]

🎉@uifabric/experiments@v6.36.0 has been released which incorporates this pull request.:tada:

Handy Links:

• Release Notes
• Official NPM Release

[msft-github-bot]

🎉office-ui-fabric-react@v6.80.0 has been released which incorporates this pull request.:tada:

Handy Links:

• Release Notes
• Official NPM Release
• Test the release with Developer Demo Site

[KevinTCoughlin]

Part of #6360