audit: Inconsistency in Quorum Modifiability

[Orland0x]

SimpleQuorumExecutionStrategy defines a storage variable for the quorum. It could be useful to
add a setter for the quorum, as it could be useful to change it depending on the circulating voting power.
Moreover, there is an inconsistency between SimpleQuorumExecutionStrategy and EmergencyQuorumStrategy in how they handle the quorum variable. In the latter it is set as an immutable. This prevents contracts deriving from EmergencyQuorumStrategy to be deployed as proxies.

[pscott]

Fix:

• Remove immutable keyword from in EmergencyQuorumStrategy
• Update EmergencyQuorumStrategy to use the initializer like Simple and Optimistic ones
• Add setQuorum / setEmergencyQuorum functions in those contracts
• Add tests for these newly added functions