This patch introduces a new `baibot_conversation_start_time_utc`
variable which indicates the time the conversation got started.
Using `baibot_now_utc` is still possible, but given that the current
time is a moving target, its use is in conflict with prompt caching.
Because the new `baibot_conversation_start_time_utc` prompt variable
is a more reasonable default, we're now using it in all sample configs.
Fallback support was intentionally removed in 9908512968,
because it was deemed OK to do so.
It turns out that Element iOS still doesn't properly do user mentions
(and likely never will, until Element X replaces it), so we can't just
drop the fallback user mentions logic without affecting all these
clients. It's possible that the Element Android is no better (unverified claim).
