Deprecates Navigation.UserMenu

[kvendrik]

WHY are these changes introduced?

I'm deprecating Navigation.UserMenu to make room for the new top bar layout in which TopBar.UserMenu will stay visible on mobile (details).

WHAT is this pull request doing?

• Adds a deprecation notice (console.warn) to Navigation.UserMenu.
• Adds a deprecation doc comment to Navigation.UserMenu.
• Adds deprecation rationale to Navigation.UserMenu's documentation.
• Adds an item to the changelog.

Before shipping: make sure the release in the docs is correct

How to 🎩

1. Copy-paste this code in playground/Playground.tsx:

import * as React from 'react';
import {Navigation} from '../src';

interface State {}

export default class Playground extends React.Component<{}, State> {
  render() {
    return <Navigation.UserMenu avatarInitials="JD" />;
  }
}

2. Make sure a deprecation notice shows up in the console.

Note: shipping this together with the frame layout changes. Version number will need to be updated before merged.

[tmlayton]

Pre-approved once #852 ships

[dleroux]

Looks good but same comments as Tim over version and wording.