Provide a data migration prompt for choosing to migrate module compendium packs during system data migration

[aaclayton]

Originally in GitLab by @unsoluble1

We spend a lot of Discord support time directing people to store their actors (and other documents) in "shared compendium" modules — getting their stuff into compendiums to reduce world load, while at the same time making them available across local worlds.

But every time a dnd5e update performs a data migration these documents are skipped, because for various reasons automatically migrating all docs in module compendiums might not be a great idea. This leaves many people's primary source of actors (etc) in an unmigrated state.

The current solution to this is to point folks at a pinned Discord post after every migrating system update, with steps to manually migrate module compendiums with a console command. This is not ideal, for I hope obvious reasons.

Proposal: When a 5e system update will be performing a data migration, provide a prompt for the GM user to select available/appropriate module-provided compendium packs to also migrate.