Change 'physical'-field on election to enum named ElectionMode

[juniwbjerde]

Now the election model has a field pysical:
physical: { type: Boolean, default: true, },
It would make more sense if this was like the type field:
type: { type: String, enum: [ElectionTypes.NORMAL, ElectionTypes.STV], default: ElectionTypes.NORMAL, ... },

(PS: the type field has been renamed to system and uses an enum ElectionSystems in the ts brach that will be merged into master when done.)

Therefore the field 'physical' should be replaced with a field named mode (or something like that) that uses an enum ElectionMode. That way it will follow the same pattern as ElectionType/ElectionSystem, and you will be able to say explicitly that an election is digital (not just physical=false)