Ensure comments fetch for KNS starts on the right page

Summary:
For a while, we've not had reviews/comments showing up for KNS content,
and ensures we initiate fetching from the correct page. It does not,
however, ensure that the number of reviews is quite correct, as
we will need to add support for replies in some way (even if it's
only done as an indication that they exist)

Test Plan:
Without patch: Notice no reviews show up, except on very-reviewed items
With patch: Notice reviews show up on all content that says it has some

Reviewers: Discover Software Store, Plasma, ngraham, apol, starbuck

Reviewed By: Discover Software Store, ngraham

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D25282