it should not be allowed to submit more attestations than vote-1

[brenzi]

Currently, meetup validation is too tolerant. Even if this is not strictly critical in terms of attacks, it made an e2e test pass that should've failed which led to trouble in production (#278 caused by encointer/encointer-wallet-flutter#866)

two options:

1. prevent submission of too many attestations directly upon submission
2. exclude attesters that submit too many attestation upon meetup validation

I strongly favor option 1) because this will allow to fix an error while it is still possible (while a gathering is still ongoing)

[pifragile]

Not happy about the label bug, it is a missing feature at most :)
Option 1 also makes more sense to me. It would be an input validation of the data. But honestly I am not sure about it, because it seems that this feature would only be here to catch errors made by the app code, and not sure if it is the job of the runtime to catch those.

[brenzi]

I think input validation is good practise, runtime or not. Avoiding hard to debug situations can save us a lot of work