`/event` does not try to backfill unknown events from federation

[MadLittleMods]

Sister issue to /context backfilling events: #3848

---

The client /event endpoint does not try to backfill unknown events from federation. "This means that if I try to follow a permalink for a room and my server doesn't have that event locally, the permalink fails. This is pretty awful UX."

• Context links give a 404 on federated rooms element-hq/element-web#1230
• Get Failed to load timeline position when click on The conversation continues here element-hq/element-web#24665 (spawned from this issue)

[deepbluev7]

It also breaks stuff like pinned events, etc.