Support time references relative to now

[dcastro]

Clarification and motivation

We should be able to parse messages such as "in 2 hours" or "30 mins from now"

In most cases, it should be obvious to participants what time “2 hours from now” will be in their timezone, so there won't be a need to send out ephemeral messages.

However, if the offset is expected to change in the designated timezone between now and then, it might be useful to inform users of this, e.g.:

NOTE: _At 01:00, the clocks are turned backward 1 hour(s) to 00:00 in your timezone (Pacific/Marquesas)

"in 2 hours"

20:30, Monday, 28 August 2023 in Pacific/Marquesas

Acceptance criteria

• when a message contains a time reference relative to now and the user will observe a clock change between now and then, we send them an ephemeral message.