Fix an invalid SQL exception that occurs when the paginator tries to
do a COUNT(*) to calculate the page count of a relation that already
includes a GROUP BY + COUNT(*) clause. We need to nest the whole query
inside a `SELECT COUNT(*) FROM (...)` subquery so the inner COUNT(*)
doesn't mess up the outer COUNT(*).
Fixes#4285.
Add <link rel="prev"> and <link rel="next"> elements to most pages with
pagination. This should work on all index pages, but it won't work for
things like pool or forum topic show pages.
Also remove the <link rel="top"> element (wasn't useful, was just a link
back to the root url).
Previously the page-based (numbered) paginator would always count the
total_pages, even in API calls when it wasn't needed. This could be very
slow in some cases. Refactor so that total_pages isn't calculated unless
it's called.
While we're at it, refactor to condense all the sequential vs. numbered
pagination logic into one module. This incidentally fixes a couple more
bugs:
* "page=b0" returned all pages rather than nothing.
* Bad parameters like "page=blaha123" and "page=a123blah" were accepted.
