Redo the generic IMAP folder separator migration
Redo the generic IMAP folder separator migration

Summary:
D2739 and D2741 have a complicated history. They originally were part of a single diff which included a migration. The migration was run but the diff was rolled back.

This has an unfortunate consequence: there's some prod shards with the columns and some more recent ones, without it.

This diff adds a new migration which adds the required columns only for the shards which require it.

Test Plan: Tested by running the migration and rolling it back.

Reviewers: kav-ya

Reviewed By: kav-ya

Differential Revision: https://phab.nylas.com/D2817

closeio/nylas,nylas/sync-engine,nylas/sync-engine,jobscore/sync-engine,nylas/sync-engine,closeio/nylas,nylas/sync-engine,jobscore/sync-engine,closeio/nylas,jobscore/sync-engine,closeio/nylas,jobscore/sync-engine