Refactor page limits to a) be explicitly listed in the User class (not
hidden away in the Danbooru config) and b) explicitly depend on the
CurrentUser (not implicitly by way of Danbooru.config.max_numbered_pages).
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.
