fixed what-if grade bug related to multiple semesters

[EricPedley]

What I Changed

Please enter a thorough and detailed description of everything you changed in this pull request.

Fixed a bug where what-if grades wouldn't enable and point total would display as because there were missing assignments from second semester, and the code that looks through the class to find assignments was checking period[0], whereas these assignments were in period[1], so I modified grades.js so it checks the assignment lists for all periods.

Screenshots of Changes

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Screenshots are required as we need to have a visual indication of what you did.

schoology.plus.grade.bug.vid.mp4

The three missing assignments you see are the ones that caused the error.

Issues Closed By This Pull Request

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Related to #228 but I'm not sure if it fixes that, because I didn't see multiple semesters in the screenshots for context in that issue.

[aopell]

Hi, thanks for making a pull request! I haven't really had the chance to look it over too thoroughly yet, but we're definitely aware this bug exists. There's a chance the fix might be a little more complex than this, but again I haven't looked too thoroughly.

I'll take a closer look soon, but once again, thanks!

[aopell]

Thanks for your changes! They look good! Sorry for the delay