Contact link for admins

In the privacy policy, you can offer contact email. it is a non-clickable string, and chatmail addresses will not actually be able to write to it. This doesn't make much sense and goes back to a time before e2ee was enforced inside the server.

We could:
- add a config.admin_contact link which can be a Delta invite link
- include a [team-bot](https://github.com/deltachat-bot/team-bot) in the deployment, and add its contact link to the page