adds deprecation notice for Navigation's userMenu prop

[kvendrik]

WHY are these changes introduced?

Follow up on #887. Deprecates the userMenu prop on Navigation as with the deprecation of Navigation.UserMenu it won't be needed anymore.

WHAT is this pull request doing?

Adds the appropriate deprecation messages to the component.

How to 🎩

1. Mount a <Navigation /> component in the playground.
2. Make sure no deprecation messages show up in the console.
3. Add a userMenu to the <Navigation />.
4. Make sure the userMenu deprecation notice shows up in the console.

Playground code

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

interface State {}

export default class Playground extends React.Component<{}, State> {
  render() {
    return (
      <Page title="Playground">
        <Navigation location="" userMenu={<div />} />
      </Page>
    );
  }
}