refactor(NodeService): Clean up getPrevNodeId() for AT/CM modes

[hirokiterashima]

Changes

• Call DataService.getCurrentNodeId() to simplify code at the beginning of the function
• Move check for mode closer to where it's actually used
• Use ternary check for application node/group node check
• Change let to const where appropriate

Test

• Going to the previous node using the prev arrow in the step tools in CM, AT, Student VLE and Preview works as before.

[geoffreykwan]

Looks good. You could probably reduce the complexity more by moving that block of code to a new function.

[hirokiterashima]

Yes, let's add it to our future refactoring todo. This is a good start to help us get there.