Add the reply fallback to the InReplyToDetails for use while loading.

[pixlwave]

Today I encountered an issue where Element X showed a loading placeholder for a reply to a message that wasn't available to my client.

Ignoring the fact that it didn't transition to some kind of failure UI, @BillCarsonFr pointed out that there's the fallback reply which is always available in an event:

Which leads me to realise that if we included the (properly extracted and formatted) fallback reply in the InReplyToDetails we could render this while loading the actual event (and if the load fails, there is still something to show).

[zecakeh]

Reply fallbacks will be gone in the next version of the spec (most likely), and removed in Ruma probably before that.

Context: MSC