Ambiguous semantics of the "url" field

I recall a prior discussion of this issue, but can't find it.

In the XML, `` is used to denote the presence of a PDF. On the paper page, it is used to (a) generate the canonical URL and (b) generate the link to the PDF. This creates a problem if there is no PDF: there will be no `` field, and therefore the paper page "URL" field is blank, [for example here](https://www.aclweb.org/anthology/2020.winlp-1.37/).

I think we should do the following:

1. Removing this ambiguity in the XML by converting all `` instances to ``.
2. (optional) Repurpose `` to list the paper's Anthology ID, and make its presence mandatory.

(2) is redundant, but is often useful in code, since a paper's full Anthology ID can be recovered directly, rather than having to be reconstructed from its ID, its (volume) parent ID, and its (collection) grandparent ID.