Refactor and/or add tests for newsletter subscription code

[toolness]

In #1556 we have a pretty big Subscribe component that currently has no test coverage. However, a bunch of its code is duplicated across two other codebases: see https://github.com/JustFixNYC/who-owns-what/pull/288 and https://github.com/JustFixNYC/justfix-website/pull/112. One way to make things more maintainable is to factor out all the common code into a separate module in our justfix-ts monorepo and then reuse it from all our codebases.