Add Newsletter Subscription Confirmation

[nring]

When the user clicks the confirmation link sent to them after signing up for the newsletter, they are added to the newsletter list and redirected to `https://engineering.deptagency.com/?action=subscribe". We don't display anything notifying the user that they have successfully signed up.

We should display a confirmation message to the user:

You have subscribed to the DEPT® Engineering blog newsletter!

TBD: Where should we display this? I think a full-page-modal link the Subscription form is overkill. I don't really want to create another component to have a smaller modal. Maybe somewhere between the header and main tagline area?

[cmdea]

Some kind of a message/toast notification in the upper right (under the subscribe button) would probably do the trick. It should disappear on its own.

[nring]

Great! The simpler the better