Fix the first section celebration modal showing logic.
On Nutmeg+ it's shown only after the page reload or after going directly
to the second section from the course home. Going through the course
with the Next/Previous buttons has no effect (which worked on Maple).
Notes:
- the weekly goal has the same showing logic, but I assume that is
correct behavior so no changes are added for it in this commit.
- showing a celebration modal for the first section completion when
going directly to the first unit of the second section seems to be a bug
(reproduces on Maple too)
cd430ebb5d