Code blocks should not be spaced 4 times, as this will
prevent the code from being colored. They should
also be spaced the same as the lists they are a part
of, to make reading easier.
Since external diffs are likely to be a bit slower than in-database
ones, add a mode that makes diffs external after they've been obsoleted
by events. This should strike a balance between performance and disk
space.
A background cron drives the majority of migrations, since diffs become
outdated through user actions.