Upgrade to Laravel 6.x

Summary: This differential upgrades us to Laravel 6.x and brings us under LTS

Test Plan: ./bin/quickstart.sh, ./bin/phpunit

Reviewers: #hkccp_developers, machniak

Reviewed By: #hkccp_developers, machniak

Subscribers: #hkccp_developers

Differential Revision: https://git.kolab.org/D994