Fix null stats.

Intent

We have faced a very strange issue where some items have null item._stats fields on unlinked tokens. This was not an issue in v10 but it seems that v11 doesn't like it. After migration to v11 and then a refresh of the page, these items disappear. Luckily, we can manually update the _stats field and that is what this MR does.

As an additional measure, we now import/export compendia items with the _stats field and have added it to all of our current pack .yaml files.

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• This is meant for a hotfix
• This is meant for the next release (see milestone)
• This needs more reviewers than normal; there may be controversy or high complexity
• This intentionally introduces regressions that will be addressed later
• There is/will be documentation changes on the wiki
• Please do not send commits here without coordinating closely with the owner
• This is a Build System change

Related Issues

• #890 (closed)

Steps to Test

1. Checkout this branch and download the world in the above issue provided by Diwako.
2. Run migration on it to v11. Check items on unlinked tokens.
3. Refresh the page.
4. Ensure that all items on the unlinked tokens remain.
5. Refresh again.
6. Double check that everything is still there.

Future Work

Find out what the hell is actually causing this issue. The _stats field is not supposed to have this kind of impact.