Add min/max duration and quorum

[pscott]

Adds min and max voting duration:

• min_voting_duration: a proposal can't be finalized before min_votin_duration has elapsed
• max_voting_duration: a vote can't be cast after max_voting_duration has elapsed

I'd like to add more tests but currently we can't manipulate the block timestamp in starknet-devnet. We might want to add some later on though :)

Closes #95

[Orland0x]

I think we should implement quorum before adding this feature

[pscott]

I think we should implement quorum before adding this feature

0503bfb :) (missing tests, they will come in tomorrow hopefully :D)

[pscott]

Latest commits add the quorum storage var. It is stored on the space, and also stored on each new proposal so that if quorum changes during proposal, it doesn't disturb any of the ongoing proposals.

[pscott]

Also added a function to edit quorum as discussed on Discord (23e86a7) :D