Add code comments about field keys

At one point, app users were called "field keys". We've since updated the UI to talk about app users instead, and we've also updated the API (e.g., the …/field-keys endpoint is now the …/app-users endpoint). However, it didn't seem as useful to change the code. That means that app users are still called field keys in the code (in both Backend and Frontend). Mostly I think that's OK, but it also has the potential to lead to confusion (as @alxndrsn has pointed out). This issue is to add code comments to Backend and Frontend to clarify that "field keys" refer to app users. For example, a comment could be added to the `FieldKey` frame in Backend.