scheduling of sessions should be independent of it's state

[Matthias Klose]

consider https://features.launchpad.net/distros/ubuntu/+spec/feistyplusone-toolchain-roadmap
which was given back to drafting, however the specs marked by XXX aren't scheduled yet; instead feistyplusone-toolchain-roadmap is scheduled again for drafting, wasting a slot and maybe preventing scheduling the XXX specs.

either have a state 'on-hold' which doesn't allow for a new schedule, or add a tag for explicit scheduling of a session. Other reasons for not scheduling a session could be the impossibility to go further on during a sprint (all things beeing discussed, but more things need to be done after the sprint ).