Add warning when permalink event ID is not found on page

Follow-up to https://github.com/matrix-org/matrix-public-archive/issues/4

Considerations

What happens if the eventId isn't found on that given day?

• With Gitter, we can easily lookup the eventId and redirect people to the correct day. But I don't think I want to do all of those round-trips to the homeserver to do this sort of thing.
• In the Matrix public archives, I think I am just going to see if the event is in the given date displayed and if not, put a little warning overlay under the room header which says "Event wasn't found on this day" and link to /:roomIdOrAlias/event/:eventId which will actually do redirection work. This way, it's a choice by the user to take the action to jump to the correct day and it's explained what happened.

Dev notes

Potential language: "It looks like we lost your position or you were given an incorrect link"

Could we tell our fault from someone giving a wrong link? Maybe by looking if the referrer is the /jump endpoint.

Start by tracing permalink handling for /:roomIdOrAlias/event/:eventId and the page's event lookup. When the event is absent from the displayed day, add the warning under the room header with a link to the event route; account for the /jump referrer question described in the issue.