Rename helper function {poll => get}CurrentJob

The actual polling is done by `$interval` -- this function itself only
does a GET once. I'm doing this for consistency with functions added in
the next commit.
Refactor the polling for a branch's sha into the applianceStatus service

...so that it can be used from the ManualController in a future commit.
Refactor back-end polling into root controller

...instead of an angular "service" that isn't tied to the model
(`$scope`).

Controllers inherit the scope from parent controllers (that is,
controllers attached to a parent node in the DOM), so this simplifies
the code greatly. If we have this DOM:

    <body ng-controller="ApplianceController">
      ...
      <div ng-controller="AutomatedController">

...then `AutomatedController` can reference
`$scope.test_pack.branches.master.sha` (from the parent
`ApplianceController` scope) in its javascript code and templates.
We don't need to communicate these values via complicated `$broadcast` +
`$on` event handlers.