[explorer] Update the txs page size list

Summary: Displaying 500 or 1000 txs on a single page is not very practical. This also makes it easier to switch to the new chronik where the paginated block txs are limited to 200 per request.

Test Plan:

cargo run

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15945