Needs investigation, but we want to decide which endpoints and how we want to expose comments along "activities"

* [https://github.com/opf/openproject/tree/dev/lib/api/v3/activities](https://github.com/opf/openproject/tree/dev/lib/api/v3/activities)


Interesting to note, the `ActivityRepresenter` ( [https://github.com/opf/openproject/blob/dev/lib/api/v3/activities/activity\_representer.rb](https://github.com/opf/openproject/blob/dev/lib/api/v3/activities/activity_representer.rb) ) already has a representation for `Activity::Comment`, although it's not backed by any model/classes. Begin of the insertion

This should be backwards compatibleEnd of the insertion