See silverstripe/silverstripe-assets#235
Makes a start at silverstripe/silverstripe-assets#219 as well

For the vast majority of sites, you really don't want to run your file migration as part of dev build.
The step is involved enough to warrant it's own task.
I don't think this is an API change, since the setting won't have affect
for anyone who has already enabled it - they would've already done the one-off migration.

See https://github.com/silverstripeltd/open-sourcerers/issues/91
and silverstripe/silverstripe-assets#235

Technically an API breakage, but I don't see migration helpers as part of our API surface.
They're one off utilitie

According to Max's feedback

Max pointed out that it's only shown on the website,
so would need HTML for the list.