* Rename local variable `highlight` to `newlyCreatedByHighlightButton` and add
  a docstring - clarify what this piece of state actually means.

* Fix `vm.isHighlight()`: It now returns `false` for new annotations that the
  user hasn't entered any text or tags for yet (instead of `true`).

* Move the code for saving new highlights to the server into a
  `saveNewHighlight()` function that's called on AnnotationController
  instantiation instead of having it in a `$watch()` function that's called
  every time the local variable `model` changes.

  Also use the presence of `model.id` to avoid re-saving highlights that have
  already been saved - instead of abusing the `highlight`
  (now `newlyCreatedByHighlightButton`) variable and confusing what it
  represents.

* Don't automatically open the annotation editor on Annotationcontroller
  instantiation if the annotation is a highlight. This fixes this bug:

  1. Create a new highlight while logged out
  2. Log in
  3. Your highlight is saved to the server, but also the annotation editor is
     open on your highlight.

  The intention is that highlights are simply saved to the server, not opened
  for editing.

  _Annotations_ created while logged out, on the other hand, are _not_ saved
  to the server on login but _are_ opened for editing on login.