[Major Purge]: Doing away with the concept of Sektions ...

Sektions will be replaced by a new concept - Clusters.

Clusters - unlike Sektions - are created / updated as needed by the
system - and not by a human actor.

Clusters are also intelligent in that they know which students
should be within them at any given point in time and what
to do with students who no longer belong to a cluster

Expect further purging of Sektions related code as and when its found.
New code - for Clusters - to follow
Getting rid of Progression

Progression tracked student <-> milestone mapping.
But given that each milestone has_one cluster - but not vice-versa -
what we need to do instead is track the more general student <-> cluster mapping.
New model and join-table in lieu of Sektions and StudentRoster