NNS1-2987: Display block timestamp in transaction history

[dskloetd]

Motivation

In order to have timestamps consistent with the dashboard, we need to use this instead of this.

Changes

Use the timestamp field if available and fall back on the created_at_time field otherwise.

Tests

Unit test added and tested manually.
Also tested that it doesn't break with an older version of the index canister that doesn't have the timestamp field.

Todos

• Add entry to changelog (if necessary).

[mstrasinskis]

Thanks!