Add honeypot for newsletter signup forms

[joshdarby]

Changes

This pull request makes the following changes:

• Adds a fake hidden name field to the newsletter signup forms to trick bots into filling it out so we know not to actually submit the form

Why

For #163

Testing/Questions

Features that this PR affects:

• Newsletter subscription forms

Questions that need to be answered before merging:

• Is this PR targeting the correct branch in this repository?
• Does this cover all forms, or are some missing?

Steps to test this PR:

1. Fill out a newsletter form as usual and make sure it submits successfully
2. Reload, fill out a newsletter form as usual and unhide the hidden field and give it a value. Submit the form and make sure instead of a real submit success message, you just see Submitted.

[benlk]

If we've got time, let's replace .uhohhoneypot with .visuallyhidden from Largo. Otherwise, this looks fine.

[benlk]

Largo has a .visuallyhidden class that can be used here instead of adding our own.

[joshdarby]

@benlk I figured Largo would have something like that, but wasn't sure if we should be concerned about using a class name with hidden in the name since I'm not sure if more advanced bots could detect that.

[benlk]

I'd go with whatever Mailchimp provides in its default form markup, since that's probably backed by a lot of research.

[joshdarby]

e19e508 switches the fields to have the .visuallyhidden class