[sources] synchronize source asynchronously when started from the UI
and redirect to the forthcoming import log, for a better user experience (there
is still a lot more to do there though).
Closes #10468967
usr/lib/python2*/*-packages/cubicweb/web
usr/lib/python2*/*-packages/cubicweb/wsgi
usr/share/cubicweb/cubes/shared/data
usr/share/cubicweb/cubes/shared/wdoc