Keep user on same page when email is entered

Right now entering email into form and submitting will redirect user to a new page:

Change this to allow user to remain on the home page and display the confirmation message as a pop up on the form itself.

Start by locating the home-page email form and the code handling its submission. Trace the current redirect and identify how the confirmation message is produced. Done means submitting an email keeps the user on the home page and displays the confirmation as a pop-up on the form.

Written by the indexing model from the issue text.