All pending migrations should run, regardless of their timestamp

[goenning]

Fider currently only run migrations that have a timestamp higher than the last executed migration.

This means we need to be extra careful when merging PRs to ensure the timestamp on the migration files are always accurate.

Ruby on Rails takes another approach where they run all migrations that have not yet been executed on the server. They still apply it in order of timestamp, but allows PRs to be merged in any order, as long as there is no dependency between the migration files.

Fider already keeps history of which files have executed, so we should be able to achieve the same.