Newsletter signup 936123

**DNM** superseding #1723 with the next step - backend for subscribing and un-subscribing from the newsletter during profile edit.

To spot-check:

0. Change `vendor/` to https://github.com/groovecoder/kuma-lib/commit/d58975d9f9ce200eac2d8c2ee1ae81aa939f0705 and update its submodules to get the updated `basket-client` library
1. Go to https://developer-local.allizom.org/admin/constance/config/ and add the `BASKET_API_KEY` from the bottom of https://mana.mozilla.org/wiki/display/websites/developer.mozilla.org+Cluster?focusedCommentId=38540359#comment-38540359
2. Go to https://developer-local.allizom.org/profile/edit
3. Scroll down to the bottom and check the newsletter form works for checking and un-checking
4. (Optional) You *may* get subscription notifications to your profile's email address, but I haven't fully debugged that