UI: Allocation detail empty state

UI: Escape hatch links on error pages

Report alloc device statistics in API and CLI

client: interpolate driver configurations

Pass service metadata "external-source" for consul UI integration

This change makes few compromises:

* Looks up the devices associated with tasks at look up time.  Given
that `nomad alloc status` is called rarely generally (compared to stats
telemetry and general job reporting), it seems fine.  However, the
lookup overhead grows bounded by number of `tasks x total-host-devices`,
which can be significant.

* `client.Client` performs the task devices->statistics lookup.  It
passes self to alloc/task runners so they can look up the device statistics
allocated to them.
  * Currently alloc/task runners are responsible for constructing the
entire RPC response for stats
  * The alternatives for making task runners device statistics aware
don't seem appealing (e.g. having task runners contain reference to hostStats)

* On the alloc aggregation resource usage, I did a naive merging of task device statistics.
  * Personally, I question the value of such aggregation, compared to
costs of struct duplication and bloating the response - but opted to be
consistent in the API.
  * With naive concatination, device instances from a single device group used by separate tasks in the alloc, would be aggregated in two separate device group statistics.

Change misleading boolean example in env stanza coercion section (#4820)

The error messages are really bad, but it's extremely difficult to
produce good error messages without the original HCL.

Also add missing SetDriverNetwork calls.

Fixes the static checks build

scheduler: Make != constraints more flexible

Show device stats in nomad CLI

This adds constraints for asserting that a given attribute or value
exists, or does not exist. This acts as a companion to =, or !=
operators, e.g:

```hcl
constraint {
        attribute = "${attrs.type}"
        operator  = "!="
        value     = "database"
}

constraint {
        attribute = "${attrs.type}"
        operator  = "is_set"
}
```