VOLDEV-4: Fix Achievements rail module button behavior

This is a fix for VOLDEV-4, implementing the following changes:
* The "Previous" button now shows up on the last page when there are only two pages (10-17 badges)
* The "Next" button shows up when cycling back to the first page in the same scenario
* A redundant Math.floor() call was removed

@Grunny @mroszka &mdash; pinging per request process