Notify slack about a new tester signing up

[plaetzchen]

I really like the idea of getting a slack notification of a new tester signing up (see #20 ) so I implemented it.

Note: I am not a ruby developer by profession so please check if this is a good idea. Tested on local though

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

:memo: Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[plaetzchen]

I signed it!

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for the commit author(s). If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.

[googlebot]

CLAs look good, thanks!

[plaetzchen]

Also tested on Heroku with our live app. Works for me.

[nickdima]

Any update on this?

[joshdholtz]

@nickdima Are you still interested in merge this? If so, can you make the requested changes and rebase? 🙃

[plaetzchen]

I think I should do this and I will. Sorry, totally forgot about this

[joshdholtz]

@plaetzchen No problem! Was just curious 🙃 Lemme know if you need any help or anything 💪

[ricburton]

I would love for this to submit to mailchimp too

[FinHorsley]

@taquitos Got time to fix these conflicts?