Add items_total to linkintegrity endpoint

[davisagli]

This will be used by the linkintegrity check when an object is deleted, to add the number of contained items that will be deleted.

It is a backwards-incompatible change, because current versions of volto expect that the result will include items only if there were some linkintegrity breaches.

Also fixes #1511 by adding the missing documentation for this endpoint

[mister-roboto]

@davisagli thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

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[davisagli]

@jenkins-plone-org please run jobs

[davisagli]

@stevepiercy Could you please review the doc updates here?

[tisto]

@davisagli am I correct assuming that we just add an "items_total" attribute to the response here? Then I'd say we can merge that and release it as a feature release.

[tisto]

@jenkins-plone-org please run jobs

[tisto]

@jenkins-plone-org please run jobs

[tisto]

@jenkins-plone-org please run jobs

[tisto]

@jenkins-plone-org please run jobs