Add a newsletter signup form

[allanlasser]

It'd be great for DocumentCloud users to learn about our "For the Record" email and have the chance to sign up for it.

Based on our handling in muckrock views.py and utils.py, we can add a NewsletterSignupForm component that collects an email users to our MuckRock-wide MailChimp list. Structurally, this should look like:

1. A form component that calls a form action subscribeToMailchimp
2. A reusable handler subscribeToMailchimp that's used in the +page.server.ts of pages that host the subscribe form.

Signups should also be tagged:

• "Newsletter For the Record"
• "Newsletter DocumentCloud"

We'll need to add a few new environment variables, which can be copied from our MuckRock Requests environment:

1. MAILCHIMP_API_ROOT
2. MAILCHIMP_API_KEY
3. MAILCHIMP_LIST_DEFAULT

[allanlasser]

@duckduckgrayduck has some concerns about spam signups to our mailing list. What are our strategies for mitigating spam newsletter forms?

Make sure we're handling errors that Mailchimp returns for spam signups.