Implement widgets for detail pages

[mpdehaan]

On each page, just as build details, it would be nice to be able to stop the build without going back to the build list.

To do this, we should add a generic facility, through the view classes, that can return custom HTML to appear below the Django Crispy Form.

Stopping builds should be a first goal, though the linked suggestion below talks about hyperlinks to related resources, which could also be useful.

The widget should be drawn so that it is not confusingly part of the forum, perhaps surrounded in some (default-collapsed) accordion.

https://talk.msphere.io/t/crosslink-variables-from-variable-sets-to-corresponding-variables-definitions/101/4