* Apply the same rounding to in message bubbles and conversation list.
Also make them consistent with Android's relative times. Fixes#682
* Show full timestamps when hovering on relative time
* Compute timestamp update delays more precisely:
Set timestamps to self-update as soon as they are able to change
rather than a fixed time since the last update.
* Refactor for customizable/localizable relative times
* Update timestamp tests
* Log timestamp update intervals to help debug #460
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