Favorites were paginating wrongly, because the pagination headers
where using the id of the id of the `Create` activity, while the
ordering was by the id of the `Like` activity. This isn't easy to
notice in most cases, as they usually have a similar order because
people tend to favorite posts as they come in. This commit adds a
way to give different pagination ids to the pagination helper, so
we can paginate correctly in cases like this.
