Code review comment for lp://staging/~adeuring/launchpad/api-query-permissions-on-object

So, here is a new version of the branch. The property
ProductSeries.user_can_set_branch is gone; instead we have a
new method SourcePackage.setPackagingReturnSharingDetailPermissions(),
which does the same as setPackaging(), but returns a dictionary with
the permission information we need on the +sharing-details page.

./bin/test registry -vvt TestSourcePackage.test_setPackaging
./bin/test registry -vvt TestPerson.test_can