I recommend that functionality in a token be required for the life of the token. If functionality isn’t required for the life of the token, then it would be better in a separate contract. Hence I believe that sale functionality should be in a separate contract to the token.
If you want a crowdsale, then you can use https://docs.openzeppelin.com/contracts/2.x/crowdsales
I also recommend considering: Points to consider when creating a fungible token (ERC20, ERC777)