Create "Notes for trainee" field for TrainingProgress which can be displayed to trainee

[elichad]

Concerns were raised about older notes being displayed to users, and the potential for CT members to accidentally write private information into the field in future. Decided to create a separate field "Notes for trainee" in the admin form, and set up so that only the notes in that field can be displayed to the trainee. The 'Administrator comments' lines will be removed from this PR and will resurface later when that field is implemented (@elichad aims to do this before the 4.2 release).

Originally posted by @elichad in https://github.com/carpentries/amy/issues/2487#issuecomment-1650272075

[elichad]

@karenword @klbarnes20 @Talishask @maneesha I told you I would aim to address this issue for the 4.2/checkout release, but I have now decided to leave this out.

The other remaining work has taken longer than I expected, and I want to get everything finalised in the next few days to have time for final testing before the rollout. I don't want to rush this feature and create potential for new bugs at the last minute.

Please instead continue with your usual workflow of contacting trainees directly to provide more information about why they are being asked to repeat/failed. I'll talk with @maneesha about how we want to prioritise this issue going forward.