Add alt_text for posts. Fixes #1086

[dessalines]

No description provided.

[Nutomic]

This will also need some coverage in api tests (by adding alt text in an existing test).

[rimu]

If I read this right, you're going to federate alt text using a field called 'name' on the Image?

The structure of the Activity is already totally different to Mastodon so copying their weird usage of 'name' seems kinda useless?

[rimu]

FYI PieFed does alt text using altText on the Image, like this:

curl -H "Accept: application/activity+json" https://piefed.social/post/39624 | jq

I hope you do the same but I'll adjust PieFed to work with whatever you decide.

[dessalines]

Currently its the name field on the attachment, similar to this: curl -H "Accept: application/activity+json" https://twit.social/@glennf/112004543167309282 | jq

[Nutomic]

Yes lets follow Mastodon on this because they are very unlikely to change their format for us. Also its better to use standard fields when possible.