WIP: parse headings containing RDFa as block_rdfa

[elpoelma]

Overview

While we technically support rdfa on heading nodes, we don't really support it in a user-friendly/good way.
To solve this issue, we could parse headings containing RDFa as block_rdfa nodes containing a simple heading node (without RDFa). An example of such a solution is included in this PR.

How to test/reproduce

• Start the test-app
• Insert html containing a heading with RDFa
• Notice that it is parsed as a block_rdfa node (containing the RDFa) with a simple heading node inside.

Challenges/uncertainties

• Currently, the changes in this PR are breaking (as the rdfa-mode of headings are no longer supported). But we could introduce a deprecation-flag or something for this change.
• We might also want to apply this to other nodes than heading.

Checks PR readiness

• UI: works on smaller screen sizes
• UI: feedback for any loading/error states
• Check cancel/go-back flows
• Check database state correct when deleting/updating (especially regarding relationships)
• changelog
• npm lint
• no new deprecations

[piemonkey]

For existing headings with links (made using the rdfaAware mode), this seems to work as expected, so I think just keeping the config option, but deprecating it, would work well to make this backwards compatible.

It seems that now there is a difference between headings that are loaded and headings that are inserted, as in order to insert a heading with an RDFa block, it's necessary to both insert the block and the heading. I think the most expected behaviour would be to parse headings created with RdfaAware: true as just a heading if they do not have any backlinks, and only with a wrapping block if they do. Alternatively, we could always insert a block_rdfa with every heading, but I think this would be more confusing...

[abeforgit]

@elpoelma this pr needs another look, I've forgotten exactly what the problem was and how the fix works.
I assume it also won't be fresh for you anymore either :)

Let me know if you happen to have a moment to investigate. No urgency, just cleaning up prs