Blog post announcing mybinder.org updates

[minrk]

We have some major updates coming to mybinder.org, waiting to deploy in jupyterhub/mybinder.org-deploy#2517

In jupyterhub/binderhub#1635, it was suggested that we should announce the changes with a blog post, which I drafted and linked from the Issue, but today realized I should have started with an Issue here for review of the post to make sure folks have a chance to look it over. It's probably more of a wall of text than it should be, but it's what I could come up with.

@manics has already given it a review and I've applied his suggestions.

Draft for review

[sgibson91]

LGTM! 👍🏻

[minrk]

Thanks! published.