Add instructions to clear out old / non-local data for 1.0 upgrade.

[dessalines]

Adds some sql commands to clear out old / non-local data.

[Nutomic]

Deleting all remote posts/comments is not a good idea. It doesnt even exclude posts which were saved, upvoted or commented by local users.

The query in LemmyNet/lemmy#6503 only deletes posts which dont have any comments, and presumaby few other interactions. Still we should mention that admins tweak these queries based on their own requirements.

[dessalines]

It doesnt even exclude posts which were saved, upvoted or commented by local users.

The exception list could start getting pretty long here.

only deletes posts which dont have any comments,

For the single person instances, even old posts with comments might not be important. But I'll add this query as an option anyway.