Add 'expired' state for TrainingProgress

[elichad]

Discussed a little in #2477, and keeps coming up as a useful feature. For example, occasionally we get old records pop up in member emails as people re-take training through a member.

We need to finalise the criteria for when progress should be considered expired. Draft criteria for completing this issue:

• 'expired' state is available for TrainingProgress
• 'expired' state is displayed in both admin and trainee views (with possible extra text for the trainee to explain why the progress is expired)
• training and checkout progress (passed or all?) should automatically expire after 18 months if an instructor badge has not been assigned
• it should be possible to either extend the expiry period in special cases, or manually change the state back to un-expired
• create a migration to mark pre-existing old progress as expired (with a cut-off at some date to be decided)
• update all relevant queries and scripts to handle the new state where needed (hopefully in many cases this can just be ignored)
• update/create automated emails with reminders of checkout deadline & progress expiry date

[elichad]

@karenword could the ITT discuss this issue at some point and try to settle on the criteria for expiring training/checkout progress?
It's not urgent, but I'd like to get the requirements nailed down before we get around to doing the development work.

[karenword]

Thanks @elichad - we discussed this today and everything you have here looks good as-is. 18 months, with options to be flexible manually. I think passed items are the only ones that should be changed -- we definitely want to preserve a record of ask-to-repeat or failed tasks.