UI: Move task group route under its own job route

Currently the router looks like this:

```
this.route('jobs', function() {
this.route('run');
this.route('job', { path: '/:job_name' }, function() {
this.route('task-group', { path: '/:name' });
this.route('definition');
this.route('versions');
this.route('deployments');
this.route('evaluations');
this.route('allocations');
this.route('edit');
});
});
```

Which mostly works but it has a couple minor issues.

1. Task groups share the same namespace as the other routes, meaning a task group named "definition" or "versions" etc. creates a route ambiguity.
2. Task groups share a loading template with jobs despite having different subnavs.