refactor(React / Ember): React as the default UI

[jjoyce0510]

Scope
This change affects folks using both React + Ember app.

Changes
This change swaps makes React the default UI and includes both code & documentation changes necessary to do so. Specific changes include

• datahub-frontend-react in docker-compose.yml, remove legacy datahub-frontend container. This means quickstart will launch with React app on localhost:9002 by default.
• Introduces docker-compose.ember.yml, an extension compose file containing the legacy ember container.
• Introduces quickstart-ember.sh, removes quickstart-react.sh.
• Doc updates to reflect legacy status of ember. '
• Changed arguments to default to React in datahub-frontend/Dockerfile.
• Swap "enableReact" gradle property to "enableEmber"

Validation
Locally built datahub-frontend-ember and datahub-frontend-react services.

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable)

[shirshanka]

LGTM