display Consul deep links

[tgross]

Having links from allocations to the Consul services which support them would make for a better user experience. To implement this we'll need:

• ui config block [feature] /ui/nomad #6413 that can provide a Consul server prefix
• API updates that surface this field to the web UI
• the web UI changes

[mikenomitch]

Since we have a Consul UI value that is exposed via a ui config block now, it shouldn't be too bad to implement this feature.

Going to add a "help-wanted" label in case anybody wants to give this a shot.

See references to "this.system.agent.config.UI" in ui/app/templates/components/global-header.hbs for a way to access these values in the UI. A person could then take this value and add the appropriate path to this URL in an external link to Consul.