Add repair page tree CLI command

[svenseeberg]

Short description

Add a shell command that repairs a tree of a given tree node ID.

Proposed changes

• Loop through all branches of a tree beginning from root node.
• Sort existing tree nodes by sorting them by existing lft values.
• Re-calculate lft and rgt values by counting through the leaves.

Side effects

• This should keep the existing structure of pages as far as possible. Some pages might be in a different order in very few cases.

Resolved issues

• https://issues.tuerantuer.org/browse/INFRA-323
• Checker for database inconsistencies #1531 (partially)

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[timobrembeck]

Awesome, this is very much needed 😍

In general, it seems to work for a few test cases I set up.
However, it did not catch problems in these cases:

• Inconsistencies between the tree_id and the parent field
• Inconsistencies between the depth and the parent field
• When the lft and rgt values are incorrect in a way that the page is not included in get_children()

I added a few suggestions that take the parent field as single source of truth and adapts all internal tree fields accordingly. I hope this works completely independent of how bad the tree structure is broken.
However, I did not validate my suggestion against the real database, so I would be very interested in the result it produces.

[timobrembeck]

I did not do super-intensive testing on this, but seems to work well for a few inconsistencies I manually created.
Would be interesting to see how this performs on the production system.
If possible, it would also be cool to create test cases from the currently broken production data, but it's more important that we finally merge this 😄