[multi-sources-removal] Drop pyrorql and zmqrql sources After a few years experementing "true" multi-sources, we're now moving to "copy-based" source à la datafeed. As pyro and zmq sources have no more known customers and the related code is in the way of future refactoring of cubicweb's core, we decided to drop support for those sources without backward compatibility. If you're still using a zmqrql or pyrorql source and you want to upgrade, ask support to move it to datafeed using a pre-3.19 version first. Related to #2919300 (first step)

action box
    Area visible in the upper left column. You have a list of available actions on the entity. The most frequently used entry is `modify`.

object
    All element created in the application
    Example: project, ticket, user, ...

relation editing module
    HTML widget that let you define new relations amongst objects.

relation
    It's a kind of 'smart' link between 2 objets of the application. It has a specific sense that determine dynamic behaviour and add a new logic of the content.