Implement sign up form in footer

[gsidebo]

Depends on #18

Like in mitxpro, we want to implement a sign up form for users to opt into an mitxonline mailing list. Part of this issue will involve some coordination with Peter/Ferdi/devops to get the correct settings in place so that these form submissions go to the right destination.

Acceptance criteria:

• Sign up form added to the footer
• Submitting the sign up form adds the user to the relevant mailing list
• Entering a blank or invalid email address triggers a validation message (this is implemented in xpro as well)
• Issue(s) created to add the correct settings for the mailing list in deployed environments (CI, RC, prod)

[gsidebo]

On hold until we determine which service we're going to use for the mailing list