add a js accesible cookie set by session_context to help users log in when 3rd party cookies are disabled...

I am starting a PR to get discussion started.

The new, tentative approach is:

1) Set a cookie on the server that can be checked on the client to see if cookies are enabled.
2) Take the correct action in the communication_iframe.

I did initial work on the client side and #2308 kept manifesting. If we are going to start transferring JS readable cookies, it may be possible to take care of both of these at once.

I have started testing an alternate approach which is to set the cookie in the client before session_context, have the server attempt to read the cookie value and send a field in its response: "cookies_enabled". This works so far in all browsers but led me down a deep dark black hole of code cleanup. I am trying to come up with a minimal changeset to make this work.

The downside to trying to set a cookie in the client is P3P on IE8. Ugh. 

Once we know definitively whether cookies can be used in the current context, we can take the appropriate action with the .watch API.