Prepare for multiple oauth channels

@vladikoff, @zaach, @ckarlof:

I closed #1423 in favor of this slimmed down version. In this version, I use @vladikoff's original PR as a basis, but remove the FxWebChannel to simplify the initial review. Once this portion is complete, I'll open a new PR with the FxWebChannel.

Here is the commit log as a preview:

> Prepare the OAuth completion logic for multiple channels.
>
> * Remove the @vladikoff's FxWebChannel for now to simplify review. I will add this in a separate PR.
> * Create a BaseChannel which is the channel interface, base other Channels off of this.
> * Extract the current OAuth redirect code into its own channel named RedirectChannel.
> * Add helper functions to service-mixin.js to clarify intentions.
> * Upon OAuth completion, display a spinner on the available submit button.
> * Upon OAuth completion, if no response is received from the channel within 10 seconds, display an error message.

Note, the channelFactory is out.

Second note: This PR is compatible with @seanmonstar's OAuth capable 123done.
Channels need to be extended to with a way to listen for/subscribe to messages. One initial use case is for the iframe flow, we need a way for the parent page to request for the UI to be shown. For the iframe flow, it should be hidden by default. I think this deserves its own issue.