fix: partially selected "Select All" checkbox state

[diogogmatos]

The partially selected checkbox state in the "Select All" checkboxes isn't working properly. Either it doesn't show as partially selected when it should:

Or it shows up when it shouldn't:

(also happens in 1st year, 2nd semester)

This is related to the "Select All" logic in the filter component, initially developed by @Carlos-oficial, and not any mistake in the event filtering itself.

[diogogmatos]

Turns out partially selected state is not showing up at all now, so it is as if that feature doesn't exist. Still a problem, just not visible to the end user.