Fix undesired behavior for back button after saving new exercise

[b-cheek]

Reproduction:

1. Create a new exercise
2. Save the new exercise
3. Click the browser back button

This will take you to the 'exercise/new' route. Maybe send them straight back to the home page, or at least clear the exercise store, warn the user, etc. Right now the back button doesn't change anything else, so the exercise is the same, so if you save again you create a duplicate exercise. See Vue router docs for some ideas on this one maybe