migrate voter management

[PascalinDe]

related to #172

Currently the add_role API is used to add voters to a form, and it is not (??) possible to remove voters from a form.

The frontend must be migrated to use the voter management API instead, and also allow to remove voters.

[ineiti]

We also have a CLI to add voters, we should either:

• remove the CLI
  or
• make it work, too

[PascalinDe]

We also have a CLI to add voters, we should either:

• remove the CLI
  or
• make it work, too

yes, good point!