UI - frontend UI proposes "cancelling a form" which is not supported by the backend

[PascalinDe]

trying to "cancel" a form from the UI leads to it running into a timeout from the backend, that does not recognize this kind of transaction

d-voting-dela-worker-0-1 | 2023-08-31T08:50:04Z WRN pkg/mod/go.dedis.ch/dela@v0.0.0-20221010131641-9c479e68be18/core/ordering/cosipbft/pbft/mod.go:639 > transaction not accepted addr=dela-worker-0:2000 reason="failed to decrypt ballots: wrong type of transaction: types.CancelForm"

[emduc]

This seems again like the same mistake from #315 (evil copy paste). This should be resolved by changing this line to the correct command in the proxy (to CmdCancelForm)
https://github.com/dedis/d-voting/blob/e8d6e50f193c296eedaaa5bc3dc0b550bf9e8343/proxy/election.go#L380C65-L380C81