- Refactor the way databases are schema-migrated so that load_migrations()
can be tested separately.
- Add an `until` argument to load_migrations() so that we can load only up to
a given timestamp.
- In load_migrations(), ignore files in which the `version` part of the file
name is empty.
- In migrations.rst, use the new, better way of ensuring post-test cleanup.
- Add tests for partial upgrades.
- LP: #971013 - schema migrations for beta 2.
- LP: #967238 - IMailingList.archive + IMailingList.archive_private ->
IMailingList.archive_policy, and add ArchivePolicy enum.
- Move the `chdir` context manager to helpers.py and add `temporary_db`
context manager.