move away from safe.contract(...) and use interface.I...(...) instead

[gosuto-inzasheru]

currently almost all contracts are initialised using self.safe.contract(...). this creates a dependency on etherscan or another explorer, and can become incorrect when the local abi db lags behind the abi of an upgradable proxy (see eth-brownie/brownie#1314).

instead, proper use of brownie's interfaces folder should be implemented. this redirects the dependency to the interfaces folder, for which the source is included in the repo and lets itself be easily adjusted. this also guarantees uniform underlying abis for all users, independent of their local brownie db.