From @raulk

Using OpenZeppelin's framework upgradable means:

• move constructor to initialize
• inherit from Initializable if it is the actual contract being deployed and inherit from onlyInitializing if it is a base class
• all of the contracts that a contract inherits follows these rules
• if user deployed have a deploy script and an upgrade script

Per https://docs.openzeppelin.com/upgrades-plugins/1.x/writing-upgradeable#initializing_the_implementation_contract
investigate if we need the constructor to have:

/// @custom:oz-upgrades-unsafe-allow constructor
constructor() {
    _disableInitializers();
}

• make initialize onlyOwner (?)
• update "initialize" method already in LinkedTokens to use new initialize method
• ensure LinkedToken owner is properly set up
• ensure deploy scripts make "msg.sender" the owner

Fixed by #841