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It was 1½ years ago we dropped Claws Mail, and we shouldn't waste time
on adapting this for the Thunderbird migration.

These used to say "Icedove (Thunderbird)" which wasn't handled so
nicely by s/Icedove/Thunderbird/.

This is just a massive 's/Icedove/Thunderbird/' both in terms of
content and filenames. No logic is changed, which should ease
reviewing of this massive commit.

Will-fix: #12242

They were not saying much that isn't said already in
startup_options#additional. So removing them will make both the
documentation tree and the integration in the Greeter simpler.

To make them smaller while keeping all the relevant information.

Refs: #12265

We have decided to remove I2P (Refs: #11276) due to our failure of
finding someone interested in maintaining it in Tails.

Will-fix: #12263

Thanks to spriver for the patch.

1. It's more useful so should be listed first.
2. We link to it from the previous paragraph so this reordering helps ensure the
   reader won't be lost while navigating on the page.