Faster and more robust user creation

[JimMadge]

✅ Checklist

• I have searched open and closed issues for duplicates.
• This is a request for a new feature in the Data Safe Haven or an upgrade to an existing feature.
• The feature is still missing in the latest version.
• I have read through the documentation.
• This isn't an open-ended question (open a discussion if it is).

🍓 Suggested change

From #2320 (comment)

Creating a single user takes approximately 90s. When adding multiple users the process can take quite a while. Also, if a single user fails to be created, the entire process will be terminated which might mean running the (long) process again.

🚂 How could this be done?

• Parallelise user creation (if possible)
• Continue adding users upon error, but accumulate the errors and report at the end of the process