Simplified lists subscribing forms

Hi!

The https://tails.boum.org/support/tails-support/index.en.html support mailing-list has a very simple way to subscribe, on the website. It would be great to have the same for -project, -testers, -ux etc.

Feature Branch: bitingbird:doc/7785-simplified_email_form

Original created by @BitingBird on 7785 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information