Add the Nomad version to the UI

To help debug, or at least quickly rule out caching issues, the Web UI should include the Nomad version number.

The only way to do this without hiding potential caching issues is to embed the Nomad version number into the [`index.html`](https://github.com/hashicorp/nomad/blob/master/ui/app/index.html) file.

If we don't do that and instead make an immediate request to [an agent endpoint](https://www.nomadproject.io/api/agent.html) (or look at the server tags on the existing servers page), then we run the risk of ruling out a caching issue when the UI assets could still be stale while the API request is current.

We could also potentially check the embedded version against the agent response to programmatically determine if the UI is stale.

Start by reading ui/app/index.html, the file identified as the place where the version must be embedded. Trace the existing UI build or version inputs only as needed, and consider the work complete when the Web UI displays the Nomad version from the embedded value without relying on a live agent request.

Written by the indexing model from the issue text.