fix: make answers past the first 999 reachable

[culfin]

Two sides of the same limit, which is why they arrive together.

In the browser. The question detail page asked for page_size: 999 and
never asked again. A question with more answers than that simply ended: the
rest exist in the database and cannot be reached from the page. It now requests
page by page, like every other list in the interface.

On the server. The template controller renders the same 999 into the page
it hands to search engines — a page of several megabytes for such a question,
rebuilt on every crawl. Capped at 100, the size the list itself uses.

How it was found, and how common it is

On a forum with 26 years of archive, where one thread has 13,368 answers.

I want to be straight about the scope: this is not visible on a young instance,
and I could not reproduce it on meta.answer.dev because no thread there is
long enough. The limit is hard-coded, though, so any instance that runs long
enough to pass 999 answers on a single question hits it — and when it does, the
answers are unreachable rather than slow.

Replaces #1573 and #1574, which are the same two changes as separate pull
requests.

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