theme_boost: Workshop menu overflows

Steps to reproduce:

1. Log in as teacher
2. Create a workshop add a workshop and tick the 'Use examples' checkbox.
3. Set the 'Mode of examples assessment' to 'Assessment of example submission is voluntary'.
4. Click on "Edit assessment form"
5. Create 3 or more aspects with "Maximum grade" 90/100
6. Click "Save and preview"
7. Try select grade from "Grade for Aspect 1" and you will observe the last values are not clickable.