eprint url aurl epub: unify handling of pre-prints

Currently we use various fields to provide a hyperlink to pre-prints. There is no perfect solution:

- `url` is not desirable in most cases because we do not want to show the pre-print link in the references list of a typical paper. But if we had a way to define a `\arxivurl` command that optionally expanded to nothing, this could be customized by users.
- `eprint` causes issues with some `.bst` styles that assume this is a reference to arXiV always.
- ~~`aurl` is ignored completely, even when rendering in HTML, which is where the information would be useful.~~ All uses of `aurl` replaced by `annotate`.
- `epub` is not standard so few .bst files support it, but it works in HTML: https://iridia-ulb.github.io/references/#FeuKleEggSprBluHut2015autosklearn

Start by comparing current uses of url, eprint, annotate, and epub in the shared BibTeX references and inspect the linked HTML example. The work is done when one documented pre-print link convention is selected and its behavior is consistent in the relevant bibliography styles and HTML output.

Written by the indexing model from the issue text.