General trustee structures from web server

[robblanco]

The Belenios web server currently supports two kinds of trustee structures: (1) a set of Single trustees, and (2) a single Pedersen group with a threshold strictly less than the number of trustees. The server is always present as a Single trustee.

It would be desirable for the web server to support the more general trustee structures supported by belenios-tool, e.g., instead of adding individual trustees to a list, allow the administrator to add either Single or Pedersen trustees, and manage Pedersen groups separately. This is primarily so that non-technical people can easily participate in elections with more complex structures, notably as trustees.

See also this thread: https://sympa.inria.fr/sympa/arc/belenios-discuss/2022-04/msg00002.html