Change Times to encompass only a single session + add migration script

[Purplegaze]

Proposed Changes

#1713 but moved to a separate target branch.

Currently breaks part of the course availability rendering, since the front-end stuff that's expecting Times to be wired in the old way hasn't been updated.

...

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Type	Applies?
🚨 Breaking change (fix or feature that would cause existing functionality to change)	X
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🚦 Test update (change that only adds or modifies tests)
📦 Dependency update (change that updates a dependency)
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[david-yz-liu]

Nice work, @Purplegaze!