[#7026] Handle detached artifacts more gracefully

On the off chance that an Artifact or AppConfig gets detached
from its project, we want to handle it a bit more gracefully.
This situtation shouldn't really come up in production, as it
means the data is bad, but it can happen on a sandbox.

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#7026] Require POST for follow/unfollow actions

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>