Add a way to exclude a scraped post from the OCaml Planet

[sabine]

It would be great if we could mark a scraped post to not be included in the OCaml Planet feed, e.g. with a flag.

Then, when reviewing PRs made by the scraper, we can exclude some posts without having them show up again on the next run of the scraper.

Context:

[yawaramin]

Fixed by #3118 ?

[yawaramin]

Maybe not...? How does the Twitter feed work? It seems to not be the same as the OCaml Planet feed, eg this is not in the scraped Planet archives: https://x.com/ocaml_org/status/1925931860367954057 ...and it mentions OCaml only once in passing at the end of the post...

[yawaramin]

Another thought: it might be better to filter in only feed posts which are explicitly tagged with ocaml. But the problem is, the feed scraper River doesn't expose categories, and its underlying library Syndic doesn't model RSS2 categories correctly: Cumulus/Syndic#85

So a post might have multiple categories like foo, bar, ocaml, and we will see only foo and thus exclude it.

Another problem is that at least several of these feeds (eg Anil Madhavapeddy, Patrick Ferris) don't actually use categories. So in those cases all their posts would be excluded. So we would probably need an override to force including posts.