Refactor logic for getting next/prev month and day on month and day views, respectively

Summary: Ref T4393, Refactor logic for getting next/prev month and day on month and day views, respectively

Test Plan: Open Calendar search, search using month view, scroll next/prev months to observe correct navigation, repeat with day view.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T4393

Differential Revision: https://secure.phabricator.com/D12723