Add backend support for plate block

[davisagli]

Fixes #1998

• extract text for indexing
• find links for linkintegrity
• add tests to show resolveuid transforms work (they were already working because we have generic support for any field named url)
• add IBlockVisitor to find sub-blocks inside a plate value

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📚 Documentation preview 📚: https://plonerestapi--2015.org.readthedocs.build/

[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!

[davisagli]

@jenkins-plone-org please run jobs

[sneridagh]

LGTM!!

no more plaintext key needed, right? I wonder if we could deprecate it too from Volto...

Now I have a doubt! Didn't we never have indexers for other blocks? for example, teasers?

[davisagli]

no more plaintext key needed, right? I wonder if we could deprecate it too from Volto...

I guess we could update the slate block indexer to collect text the same way.

Now I have a doubt! Didn't we never have indexers for other blocks? for example, teasers?

It seems we do not. In the case of teasers, it's hard to say what is the right thing to do: including the teaser title in the searchable text could make it harder to find the actual page that is teased, and it would not necessarily be up to date when the teaser is dynamic.

btw, I realized there is one more thing I need to do: add an IBlockVisitor that can find sub-blocks inside a plate value.