Replace GPLv3 with AGPLv3 license.

Starting from this commit, the MDwiki sources are not licensed
under the terms of the GPLv3 anymore. Instead, the AGPLv3 applies
with the same additions to the license as before.

This also applies to all external contributions, as the license
change is subject and in conformance to the CLA.
Allow a stage to subscribe to self
Add tests for stage/stagechain