[8] Ids removed in span elements when they shouldn't

[orballo]

In this particular case, the span within the h3s should have an id, but the id is not in the REST API content.

This would be a good moment to implement custom allowed attributes in HTMLPurifier. They could be sent as a query or set in the plugin settings.

https://docs.google.com/spreadsheets/d/1FWcvX7KiLAppGW2qOICxs8ATWqV1uEsCVdTnq5mK7q8/edit#gid=0&range=A41

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#cs: @SantosGuillamot
#supporter: @orballo

https://frontity.pipedrive.com/deal/263
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