Rename fallback_id to default_id

Output of the meeting on 2020-11-10: the term fallback_id is mostly irritating, as it was initially thought to handle default playlists, if no other playlist is set (as a standard mode of operation). Only when a station fallback is active we want to treat it actually as an error.

Therefore we should rename the fallback_id fields in steering to default_id for each "show" and "timeslot".

Would be good to already change in Aura 1.0, but can also be moved to 1.1 in case of doubt. Whenever this is closed a dashboard and engine ticket (with the same milestone) should be opened to adapt to the new change. Any other thoughts on that @david @pretzelhands ?