Load messages after the given day

[MadLittleMods]

Following up with the UX problems mentioned in #71, it would be really nice to be able to load messages after the given day:

UX problems

We can't show messages after the selected day

With /messages, we can only pick an event and paginate backwards because paginating forwards does not work properly with /backfill. And /context doesn't /backfill at all.

Because we can't get messages after the selected day, if the day doesn't have enough messages to fill up the screen, the URL will immediately change to a day before and there is no way to scroll low enough to select the day again except for the calendar.

-- #71

Todo

• Create MSC for being able to /backfill forwards -> MSC4000
• Maybe update /context/{eventId} to backfill and with the MSC, we can backfill both forwards and backwards from the given eventId. We can then use /context instead of /messages.

[MadLittleMods]

Given how the archive now works by showing the end of the date/time specified in the URL looking backward up to the limit, it's less clear that we actually want to do this.

The original "want" and desire for this was to be able to render the top of the date/time specified looking forwards but that just doesn't work right now because it never backfills over federation. And because we were rendering from the top, our scroll would be nicely just at the top of timeline (no monkey business to scroll to the bottom) and the date header would be visible regardless of whether we made it sticky.