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.
Closes #1851 (closed)