Return a cryptographic signature of the user id, associated to the selected poll and optionally to the specified keyword.

This signature can act as a proof for the users to guarantee they have an activated account, and optionally that they meet specific requirements that can be poll dependant.

The query parameter keyword identifies the requested proof, and its set of requirements that are expected to be met by the logged-in user. Each set of requirements includes at least having an activated user account.

List of accepted keywords and matching conditions:

"" (empty): Same as "activated".

"activated": Only check if the user has an activated account.

"proof_of_vote": Additionally check if the user has participated in
                 the selected poll.