Rethink data structures for offering / template proposals (arrays > mappings)

[adamdossa]

Change these variables from being mapping(address => address[]) to mapping(address => (mapping address => bool)) and then allow the frontend to cancel based on just knowing the templateProposal / offeringContract addresses.

[pabloruiz55]

We could probably have both so we can still retrieve a list of all template proposals associated with a ST.