[Signup] Add optional subscription email and community links during signup

[srinjoyc]

Why?

We deliver a lot of information through developer email subscriptions to let developers know about updates, events, etc. If a developer has gotten to the stage they are signing up for a testnet faucet account, they may be interested in subscribing to this information but likely have not come across a place to subscribe yet.

What?

Add an optional form underneath the registration form where the user can opt-in to the dev updates email subscription.

@githubwei let me know if anything needs to be changed or if anything else should be here