Show offline entity backlog events and unknown events

[ktuite]

Related to backend PR getodk/central-backend#1332

It used to be that any unknown action defaulted to showing up as a comment. I've changed it to make it show the text of the action... and also a clock icon. It should probably be a more generic icon, but the clock goes with the specific "unknown" audit actions I'm targeting but don't feel like making special cases of yet.

What has been done to verify that this works as intended?

Why is this the best possible solution? Were any other approaches considered?

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