Openedx-filters

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Added
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* VerticalBlockChildRenderStarted filter added that is called when every child block of a VericalBlock is about to be rendered.

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Added
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* Cohort assignment filter to be used with every cohort assignment.

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Changed
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* Change dashboard/course about render exceptions naming for clarity

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Changed
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* Remove CourseHomeRenderStarted since it's not going to be used.
* Change RenderAlternativeCertificate to RenderAlternativeInvalidCertificate.

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Added
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* More significant exceptions for template interaction.

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Added
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* More significant arguments to the certificate creation filter.