composability: refactor democracy using `Origin` instead of nested dependencies

[brenzi]

Our pallets have deeply nested dependencies which means that they are not composable. i.e. democracy logic can't be used by non-encointer runtimes. Moreover, coding gets trickier the more dependencies we add.

This should be avoided:

pallets/democracy/src/lib.rs

Lines 85 to 90 in 557aea1

	pub trait Config:
	frame_system::Config
	+ pallet_encointer_scheduler::Config
	+ pallet_encointer_ceremonies::Config
	+ pallet_encointer_communities::Config
	+ pallet_encointer_reputation_commitments::Config

We could introduce a custom Origin per CommunityIdentifier (or global) which is allowed to enact proposals by calling specific dispatchables.

As we are in pragmatic mode currently, this can surely wait. But a cleanup is due sooner or later