Implement "Online" tab on the Nodes page

[frol]

Story

Some of the users check and confirm that their node is up and running with the network using the Online Nodes page.

Acceptance Criteria

• The tab has the number of online nodes
• The tab features the telemetry data:
  • node id
  • latest telemetry update
  • latest known block
  • agent information (name and version)
• Mobile ok
• Tested

Assets

• Mockup: https://www.figma.com/file/CGGQaZEXTxl37ItJW6DlDD/Validator-Dashboard?node-id=0%3A1

[frol]

I believe this is not that important since we exposed all the staking pools on the list (#661) and non-staking-pool online nodes do not provide much of the information that makes sense to be displayed in a table view. The online map will make more sense, but that is a separate issue.