Highlight when message timestamps look wrong

Ideally servers wouldn't fall over such that messages are significantly delayed, however that's not how computers work. We should highlight the fact that a message is more than 5 minutes late by our clock to warn users that the messages are old.

Synapse already tracks the received timestamp - maybe we just need to expose it through the spec?

Start by checking how Synapse’s received timestamp is represented and how the relevant spec could expose it, then trace message timestamp rendering in Element clients. Done means messages more than five minutes late by the user’s clock are visibly highlighted across the affected spec and client work.

Written by the indexing model from the issue text.