Add back mozcrash feature

[whimboo]

With the rewrite of the extension the feature to crash Firefox had to be removed. We have to check what's necessary to get the code which is using ctypes to get running again.

[kyoshino]

Just heard from @SeburoSUMO that Firefox 62+ comes with about:crashparent that triggers a crash of Firefox (Bug 1462702). We can probably include a link to this page in NTT.

[whimboo]

Note that there is also about:crashcontent for the content process. So it indeed means that we can bring back the crash feature. I would find it helpful if we could make it that users do not actually see which pages are getting loaded.