attach(position, adjacentRelation) {
    this.from.markDirty();
    this.addToOutgoingRelations(position, adjacentRelation);
    if (this.to && this.to.url) {
      this.refreshHref();
    }
    return this;
  }
Attaches the relation to an asset.

The ordering of certain relation types is significant
(`HtmlScript`, for instance), so it's important that the order
isn't scrambled in the indices. Therefore the caller must
explicitly specify a position at which to insert the object.

@param {String} position `"first"`, `"last"`, `"before"`, or `"after"`.
@param {Relation} adjacentRelation The adjacent relation, mandatory if the position is `"before"` or `"after"`.
@return {Relation} The relation itself (chaining-friendly).