Use first sorted item in Compare view as 'firstCompareItem' rather than first unsorted item

[FlaminSarge]

Also confirmed that if the sorting is changed it updates properly (when initialItem is no longer in the list).

Initial state:

Previous behavior after removing first item:

Updated behavior after removing first item:

[bhollis]

If the initial item is missing, we probably shouldn't show the first button at all - there's no longer a reasonable "comparison exemplar" for the session, and choosing any of the remaining items is as good as any.

[FlaminSarge]

If the initial item is missing, we probably shouldn't show the first button at all - there's no longer a reasonable "comparison exemplar" for the session, and choosing any of the remaining items is as good as any.

I'm not so sure on that, I've definitely had a use case where I removed the initial item to check something and then 'reset' the compare window using the first button. I feel that if a user starts a compare session they should be able to get back to initial session state without having to find the item in vault to start Compare again, and the first button handles that pretty well.