Fix "list quarantined media" API to have semi-stable ordering

[turt2live]

Note: This cleans up previous code introduced by #19268

When first writing the API, "unquarantining" media was not properly considered. If media was unquarantined and an application was treating from tokens as long-lived when listing quarantined media, the endpoint could skip rows like so:

1. Media A is quarantined
2. Media B is quarantined
3. Application lists quarantined media, caches from=2
4. Media A is unquarantined, shifting B from row 1 to 0
5. Media C is quarantined, becoming the new row 1
6. Media D is quarantined
7. Application lists media with from=2, gets [D]

To fix this, we invent a pagination token which uses time and a relative index. It's not super stable still because the relative index can still change, but it's likely stable enough for most usage (iterate as fast as possible to the end).

If an application requires a proper time-based stable token, it can generate a timestamp then append -0 to it to set the relative position to the zeroth row. This may return rows the application has already seen, as described by the admin API docs. This particular method of generating the timestamp manually is not documented because it's not as stable as relying on the last seen next_batch's internal timestamp.

This PR doesn't shift the whole endpoint to timestamp-only tokens because the prior PR populates rows with 0 for a timestamp, which may span thousands (or millions) of rows, breaking the ability to use limit properly.

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[turt2live]

I do accept that this is custom, weird, and not-quite how pagination works. My defence is I have a custom, weird, not-quite-paginating use case 😇

(though, if someone more qualified wants to make this use PaginationHandler instead, I'd happily close this PR in favour of that one)

[turt2live]

We probably don't need this backwards compatibility given the PR which introduced the endpoint has only been on develop for a few days as of writing. I've kept it anyway because I honestly just don't want to fight the unit tests too much. I can be convinced to enter battle, however.