Rules approval for new users

[noplanman]

The rules need to be approved (in a private bot chat) to get the permissions necessary for posting in the group.

At the moment, a simple click on an "I Agree" button is all that is needed.

todo:

• Kick user after a defined expiry period.
• We'll test it like this, and if need be because of excess spam users, create a more complex activation.

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Initial rules message, showing the "I Agree" button

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After clicking "I Agree", an alert is displayed and the button now links to the group.

Closes #19