partial fix #5156: Pending changes and logging of renumbered XREFs

[bschwede]

Partially refers to #5156:

• Only executes data fix when there are no pending changes, in order to prevent breaking relations.
• Logging the mapping of renumbered old and new XREFs to table change.
• Knowledge of modified XREFs is the basis for providing additional data fixes - e. g. through a custom module.

[fisharebest]

There are two separate changes in this PR.

The check for pending changes is mostly OK. Instead of redirecting to the pending-changes page, I prefer to give a link to it. Less surprising for users. I have added this in 0d4706e

I'm not sure about the other change. I don't like abusing the changes table like this. In a future release, the changes table will be removed.

We could add a "full text scan" option to the renumber-xref logic. But I fear it would be extremely slow.