Arbitrary collections

In ACL's early days, there was a newsletter attached to CL called "The Finite String". [We have ingested these piecemeal](https://aclanthology.org/search/?q=%22the+finite+string%22) with CL issues, but there is no easy way to surface them as their own volume or collection or display them together except through a static HTML page. Adding a new `finite-string` tag won't work, because these are paper-level, not volume-level.

I'd like to have this ability. Two ways I can imagine implementing this:
* Virtual collections, where we could create a collection that would host an arbitrary collection of papers and volumes.
* Tags, which could be added to any paper and then gathered and displayed under the tag name.

I welcome thoughts on this idea and any foreseen technical difficulties. It seems like tags might be the easiest thing to do.