Private fields in `get-application-for-user`?

In PR #2651 (implementing #2453), it would've been nice to be able to add some fields to `get-application-for-user` that wouldn't get output from the GET /api/application/:id API.

For the not-customized-per-user applications we have two different formats: `get-application-internal` and `get-application`. Perhaps we need to add `get-application-internal-for-user`? Or is it too many different formats?

Another option would be to strip out keys from the application on the API level. However that might make it hard to know what's being hidden and what's not, leading to leaks of internal information.

Start by reading PR #2651 and issue #2453, then compare the get-application-for-user, get-application-internal, and get-application formats mentioned here. Decide whether private per-user fields need a separate format or explicit API filtering, with completion defined by preventing internal information from appearing in GET /api/application/:id.

Written by the indexing model from the issue text.