cubicweb-blog #1251814 add the notion of continuation for a blog entry [open]

Sometimes, we want to talk about a big topic. For that, we commonly split the topic in several parts. It will be useful to have a relation to indicate that a blogentry is the continuation of another blog entry or that another blog entry is the continuation of the current one, in order to have a link in the blog entry view to see previous and next blogentries linked to this one (in the good order).

done in<not specified>
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