Override, grant and revoke for admin

[alexkeating]

Add this take from here to prevent the admin role from being changed or revoked.

    ///
    /// Override AccessControl role management
    ///

    /**
     * @dev See {AccessControl-grantRole}. Reverts for `DEFAULT_ADMIN_ROLE`.
     */
    function grantRole(bytes32 role, address account) public virtual override(AccessControl, IAccessControl) {
        require(role != DEFAULT_ADMIN_ROLE, "AccessControl: can't directly grant default admin role");
        super.grantRole(role, account);
    }

    /**
     * @dev See {AccessControl-revokeRole}. Reverts for `DEFAULT_ADMIN_ROLE`.
     */
    function revokeRole(bytes32 role, address account) public virtual override(AccessControl, IAccessControl) {
        require(role != DEFAULT_ADMIN_ROLE, "AccessControl: can't directly revoke default admin role");
        super.revokeRole(role, account);
    }