[explorer] Fix sorting txs by age/date

Summary:
The table sorts the rendered data, i.e. the age/date string. Use the timestamp instead.
Note that due to pagination this doesn't sort over pages, but only inside the current page.

Test Plan:

cargo run

Open an address page and check sorting by age and date works.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D17562