[server] Introduce migration that adds baseRoutePath to AppURLFacts

Summary:
This should work in both dev and prod environments.

Depends on D3547

Test Plan:
We tested this migration extensively:

1. We tested it on the original state I had in my dev environment
2. We made sure it's idempotent by deleting the migration checkpoint and re-running after the files had been mutated
3. We also tested it with what we thought we have in prod

Reviewers: varun, atul, palys-swm

Reviewed By: varun

Subscribers: Adrian, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3548