Restrict access for creating proposals

[matt-user]

Description

Only allow authorized users (Identities) to create proposals to a DAO.

Motivation

Currently a user could create malicious proposals with a small acceptance percentage and pass the proposal.

Specification

When a DAO is initialized pass in a list of authorized Identities which can create proposals. Then when a user tries to create a proposal check the senders Identity and require that it matches one of the authorized Identities.

Notes

Later on we can add some other method for adding proposals that is more decentralized. Or another possibility is only allow users with a certain amount of governance tokens to add proposals.