Add Newsletter signup form

[nickoneill]

We have a very popular newsletter signup form on iOS that appears above the list of issues. Let's do the same thing on android.

This form appears by default for new users and can be dismissed immediately with the "no thanks" button, or if a user submits their email.

Some features:

• dismiss button that prevents the form from showing again
• some basic email validation
• replace the subhead text with an error if the email is invalid or there's another problem sending the email

Here's the POST request code for submitting the email in swift, we should do the same but replace the tag with android:

var req = URLRequest(url: URL(string: "https://buttondown.com/api/emails/embed-subscribe/5calls")!)
req.httpMethod = "POST"
req.setValue("application/x-www-form-urlencoded", forHTTPHeaderField: "Content-Type")
req.httpBody = "email=\(email)&tag=ios".data(using: .utf8)
URLSession.shared.dataTask(with: req).resume()

Here's what a few form states look like on iOS:

[dektar]

Screenshots of how this could look before and during scroll. I haven't yet wired it in to make the newsletter POST request.