Use ->addMonthsWithoutOverflow() and ->subMonthsWithoutOverflow()

Summary: Carbon::now()->subMonth(1) for 2020-03-30 will end up as 2020-03-01, which simply isn't a month.

Test Plan:
phpunit failed.

apply change.

phpunit succeeds.

Reviewers: #hkccp_developers, machniak

Reviewed By: #hkccp_developers, machniak

Subscribers: machniak, #hkccp_developers

Differential Revision: https://git.kolab.org/D1096