Simplify timestamp formatting

[Kijewski]

This PR makes the tool use tz-rs to convert a timestamp to the user's local time zone, and respect changes to their DST since the creation of a device. The relevant time zone designation is printed in the resulting string.

This PR further removes the usage of unwrap() from formatting a timestamp. On error Debug::fmt() is used to express the timestamp.

And lastly the PR makes divining the user's timezone lazy and caching.

Resolves #82 Resolves #83

[Kijewski]

I'm not actually a user of this fine tool, so the PR is totally untested. :)

[skeet70]

+--------------+-------+--------+----------------------------------+---------------------------+---------------------------+
| Group Name   | Admin | Member | Group ID                         | Created                   | Updated                   |
+--------------+-------+--------+----------------------------------+---------------------------+---------------------------+
| test-group   | ✓     | ✗      | f3def3ae7103263736d2bfcbfdf0b69a | 2022-04-14 15:30:33 (MDT) | 2022-10-27 12:31:15 (MDT) |
+--------------+-------+--------+----------------------------------+---------------------------+---------------------------+
| test-group^2 | ✓     | ✓      | e3fd3372bdb1c5dfe2ba8c6ead1fa253 | 2022-04-14 16:51:52 (MDT) | 2022-04-14 16:51:52 (MDT) |
+--------------+-------+--------+----------------------------------+---------------------------+---------------------------+

+---------+------------------------------------+
| Group   | test-group                         |
+---------+------------------------------------+
| Admin   | ✔                                  |
+---------+------------------------------------+
| Member  | ✖                                  |
+---------+------------------------------------+
| Created | 2022-04-14 15:30:33 (MDT)          |
+---------+------------------------------------+
| Updated | 2022-10-27 12:31:15 (MDT)          |

Works, and thanks, we appreciate the help!