admin transfers page: db paging update

[monkeyiq]

This is an update to #1425

Thanks @makinog3 for the above PR which inspired me to tinker on these updates!

It allows a fake $total_count if the offset is zero and there are no real selectors in the SQL query. This is the case for when you first view the admin / transfers page and the totals are likely to be large, somewhat uninteresting, and maybe slow (seq scan on the transfers table).

I have also limited the scope of the for clause to only consider pages near the current offset rather than from 0 to $total_count.

[github-actions]

If there are selenium UI results for this code they will be at filesenderuici@4aa4a3b

[monkeyiq]

On further thought the $total_count might not be needed. That query really just wants to know if there are entries before and after the $offset. Something like the following but with bounding for ensuring that offset does not go negative. Seems like a good idea for a subsequent PR at some stage.

        'count'  => $page_size * $display_page_num,
        'offset' => $offset - $page_size * $display_page_num/2,