Create a tag in an organization, giving it a name and a color.
Names must be unique within the organization, and `color` is a six-digit hex triplet. Requires the `posts:write` scope.

```graphql
mutation CreateTag {
  createTag(input: {
    organizationId: "some_organization_id",
    tag: {
      name: "Summer sales"
      color: "#F523F1"
    }
  }) {
    ... on TagActionSuccess {
      tag {
        id
        name
        color
        colorName
        isLocked
      }
    }
    ... on MutationError {
      message
    }
  }
}
```