Find out what an event is about

**Tension**
When I enter an event, for example smarketplace demo, I would like to be able to enter the event's [Subject](http://virtual-assembly.org/ontologies/pair-2021-summer/index-en.html#Subject), i.e. the **project** Smarketplace, in addition to entering a topic.

**Proposal**
In the PAIR ontology, there's a dedicated [Object Property: about](http://virtual-assembly.org/ontologies/pair-2021-summer/index-en.html#about)
We'd like to add it so that we can link events to subjects such as projects, orgas and so on.
What's missing, however, is the inverse relationship that would make it possible to identify when you're on a [subject](http://virtual-assembly.org/ontologies/pair-2021-summer/index-en.html#about: the events or ideas they're about. The label could be something generic like: "About ..." / "A ce sujet"

A description of alternatives you've thought of.
Not really .

No files, tests, or entry points are named. Start by locating the PAIR ontology models for events and subjects and inspect the existing `about` property; done means events can link to subjects such as projects or organizations, and a subject view can identify related events or ideas with an appropriate label.

Written by the indexing model from the issue text.