FE posts MV options to /features/undefined/mv-options/ when creating a new feature

[matthewelwell]

To reproduce:

1. In a browser with the network console open, create a new MV feature with at least one variation
2. Observe that the FE makes a post request to /features/undefined/mv-options/ when creating the variations

This highlights 2 issues:

1. The FE should be posting to /features/:id/mv-options/
2. The API should be validating the feature id given in the URL path

Item (1) will need to be resolved before item (2), or creating MV features will be broken.