Mastodon 4.3 introduced a new API to fetch a timeline of posts that
mention a trending link.
Use that to display a "See <n> posts about ths link" message in a
trending link's preview card (if supported by the server).
Define a new timeline type with associated API call to fetch the
timeline.
Add an accessibilty action to support this.
While I'm here also support author's in preview cards that don't have a
related Fediverse account; show their name in this case.
Fixes#1123
57be148fbf