Use ERC721Holder & ERC1155Holder for Governor, Timelock

[frangio]

As discussed before in #3174, the Governor and TimelockController contracts are able to receive ERC721 and ERC1155 tokens, so they should implement the respective Receiver interfaces. This was fixed in #3230, but due to storage layout compatibility (#3174 (comment)) we didn't use ERC721Holder and ERC1155Holder. We should do that now.

[kimanikelly]

@frangio Is this issue still relevant to be picked up? If so, I'd be glad to start working on it