Content directory smart contracts: Storage contracts

[Lezek123]

• Create ChannelStorage, CuratorGroupStorage and VideoStorage contracts according to Redux: Requirements for smart contract content directory #1520 (comment)
• Create Solidity interfaces for each of the storage contracts, so that if we ever change given storage contract implementation without changing the interface we can assume it will be compatible with current logic contract / tests / frontend expectations.
• Create an abstract contract like InitializableStorage that the upgraded storage contracts can extend in order to faciliate possible storage migrations