fix: merge course labels across pages

[ishita778]

Fixes #533

Before:

After:

example search URL: https://utdirect.utexas.edu/apps/registrar/course_schedule/20249/results/?ccyys=20249&search_type_main=FIELD&fos_fl=C+S&level=U&x=29&y=18

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This change is 

[doprz]

Please make sure that all checks are passing

[Samathingamajig]

The current version will always remove the first row of a page (except for the first page, which doesn't go through the addRows function), because a header row has row.course === null, which is true on the first iteration, and then each course object with non-null values have unique places in memory so !== will always be true.

When you go to the page for unique numbers 00000-99999, you can see where this logic fails.

The first 2 removals of "the first header on the page" work because they happen to be the same as the previous label

However, for this example, the page boundary lies cleanly between two different courses, so this label should be preserved

despite the fact that it should be preserved, this PR deletes that label (it should be between 00875 and 00880

You'll need to query on the row.element dom node if course === null to get the course title, then check it against the previous.

To preserve data across multiple calls to addRows, you should use useRef(https://stackoverflow.com/a/56456055/12101554) because you want to store state but you don't need it to trigger a rerender when you change lastCourse.