add WhatsApp support

[git-sid]

Is your feature request related to a problem? Please describe.
the telegram bot was a success, but many noticed the lack of a whatsapp bot.

Describe the solution you'd like
WhatsApp is a very popular instant messenger that has a lage userbase. Therefore, a whatsapp bot would fit the need of many who don't use telegram/twitter/...

Describe alternatives you've considered
We don't have to reinvent the wheel. Bridges between e.g. telegram and whatsapp exist. If there's something already that fits our needs, we could use that.

Additional context
I'd need some support. Let's talk @b3yond .

[b3yond]

Use flow:

1. The admin sets up an account with a burner SIM card and yowsup-cli register: https://github.com/tgalal/yowsup/wiki/yowsup-cli-2.0
2. When users want to subscribe or report, they write to the mobile number via Whatsapp.
3. The bot ignores what they first said, and asks them, which city they want to join.
4. The user joins the city with a very specific command, e.g. "/join berlin".
5. From then on, the user receives reports, and can send them.
6. The bot keeps track via the database, which user is in which city.

Whatsapp is nasty >.<

[git-sid]

short reminder:
WhatsApp doesn't like bots and therefore bans them regularly. We could try a POC to see how fast it could happen. Using that information, we should decide whether to implement WhatsApp support or not.

[b3yond]

Maybe we should wait with this until we have implemented the architecture rework?

[git-sid]

Maybe we should wait with this until we have implemented the architecture rework?

Exactly my thought. We should prioritize #79