Don't call mount/3 of LiveView when the instruction of returns of hooks on mount is :halt

[nallwhy]

It fixes #1598.

If you agree with this approach, I will keep work on it.

TODO

• Add tests

[mcrumm]

@nallwhy Looks good! There is already a test for redirect on mount, but we can update it to ensure the mount callback is not invoked on the LiveView.

In the test support lifecycle module, look for RedirectMount and add the following mount callback:

# Phoenix.LiveViewTest.HooksLive.RedirectMount

  def mount(_, _, socket) do
    case socket.assigns.live_action do
      :halt -> raise "mount should not have been called"
      _ -> {:ok, socket}
    end
  end

then once the tests are passing we can

[mcrumm]

I am working on another fix too, so I went ahead and merged this with the test changes, thanks so much!