Follow-up from "Optimize grading ui"

To-do: Make sure the history dropdown is always there, but has a better empty state.

cc @dzhuang

The following discussion from !27 should be addressed:

• @inducer started a discussion: (+2 comments)

  This change is what made me believe that this was related to (or replaced) the grading history. If I see correctly, all this does is wrap that in {% if prev_grades %}, right? I'm not sure I like that, because this UI element is less discoverable if it's only there occasionally.