Init cleanup

@phgross @lukasgraf @Tschanzt After migrating a site, we often have to run different cleanupsteps (linkcleanup, special dateupdaters, adding special portlets...). At the moment I abuse the transmogrifier to do this steps. This should be implemented better and easier.

One possibility is to put this cleanups into a pythonscript and run them with bin/instance run. This is not very nice and makes the cleanups more difficult to run.

A better possibility is, to handle this with a cleanupadapter. We should overwite the 'jsonmigrator' view (or add a separated view) to display the registered cleanupsteps. The 'manage-upgrades'-view of ftw.upgrade is a good example how it should be.

Some inputs? hints?