Update some calls to/usage of getWithChildren

[nfebe]

The back-end currently allows api/folder/getWithChildren to be called with an array of folder VOs. It is not clear whether this ability is important in web-app, however it does not appear to used in practice some have some code references such as this one which @liam-lloyd found :

web-app/src/app/shared/services/data/data.service.ts

Line 257 in 8f3402b

promises.push(folders.length ? this.api.folder.getWithChildren(folders) : Promise.resolve());

• We have agreed that api/folder/getWithChildren that it is not ideal for this to expect and return children for more than 1 folder.
• It might not be relevant in wep app after all.

This issue is then to check and make sure that:

1. Such behavior is not needed in web-app and if so, the current references removed.
2. Back-end's api/folder/getWithChildren is updated to never expect more than one folder if 1) is true.