Rename federation fields for multi-community

[Nutomic]

For community use content for sidebar (markdown, long text) and summary for short description (150 chars, plaintext).

For multi-community use preferred_username, name in the same way as community. Use summary for the short description. Note that multi-community currently doesnt have a sidebar with markdown/long description and so no content.

Tested this with Lemmy 0.19 and the sidebar gets set correctly. However it also uses the same sidebar text (from summary) as short description, which is shown with HTML tags. So we should ignore summaries which contain <p>. With communities fetched from Piefed its the same. NodeBB doesnt have source, so if we filter out summary containing HTML we wouldnt get any description text. Peertube also uses summary for the long description (seems to be plaintext).

To be consistent, person.summary should also be called content. But in fact Mastodon uses summary with HTML content (and no source field).

Overall I'm unsure if this is the right direction. It results in incompatibility between different versions and platforms which cannot be resolved cleanly.

Requires #6295

TODO:

• update examples