Federated Activities should use ID instead of URL

[trwnh]

Ideally Mastodon would support Update activities, but there is another solution: federation permalink can be ID-based instead of title-based, and a "pretty url" can be formed from a custom slug containing the title or whatever the user enters.

So, for example, I might post an article on Plume that federates out as https://plume.example/~/trwnh/6743826481264 and the title is test post. The default pretty-url slug would be test-post (but I could edit it manually to test if I wanted to, or 2018/10/11/1, or 2018/testpost, or whatever I wanted) and then the URL would become https://plume.example/~/trwnh/test-post (or whatever i set it to)

When federating that out, you would use the id as that permalink, and url as the slugified link. So for example:

"id": "https://plume.example/~/trwnh/6743826481264",
"url": "https://plume.example/~/trwnh/test-post",

And the content of the Create activity would include the id instead of the url. So something like <p>$title - $id</p> at the simplest, and then it would show up in Mastodon like this:

trwnh (@trwnh@plume.example) posted 23s ago:
-----------------------------------------------------------------
test post - https://plume.example/~/trwnh/6743826481264
-----------------------------------------------------------------
[reply] [boost] [favorite] [menu]

or trwnh posted a new article:<br>$title<br>$id, and then it would show up in Mastodon like this:

trwnh (@trwnh@plume.example) posted 23s ago:
-------------------------------------------------------
trwnh posted a new article:
test post
https://plume.example/~/trwnh/6743826481264
-------------------------------------------------------
[reply] [boost] [favorite] [menu]

The important thing is that the URL that gets federated out cannot change, so it's a bad idea to use a value that can be changed. Not every implementation might support Update activities, so using the id is more robust and will fail in less cases.

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Related: #221 #207