Add "Proof of concept" label

Purpose:
To emphasize that the UI implementation of Inventory-es is a proof of concept the appropriate label will need to be added the left pane.

Scenario:

• Given the Inventory-es app
• When users opens the landing page
• Then "Proof of concept" label is visible on the top of the left pane under Search and Filter label
  

Note: Elasticsearch logo as presented on the mockup is not in scope.