DB cleanup script

[cwarwicker]

Write a script to clean up some old data in the database we don't need any more, for example old sprint_users records.

Also explore if there's a way to reduce the size of the goals history table, as it is by far the largest.

[cwarwicker]

Done this for sprint_user records. Need to look at goal_history as well.

[cwarwicker]

Also need to clean up sprints which never ended which are sitting as completed=0 in the database.

[cwarwicker]

^ that is done now in f18e27a