Fix lecture edit page if active term is not set

[fosterfarrell9]

This fixes #682 by adding the missing case in the older_than? method in the Lecture model.

[Splines]

LGTM.

• Maybe you can add an end-to-end test to ensure that the lecture#edit page looks as expected? (instead of showing an error?)
• And maybe even add a unit test for the lecture.stale? method to ensure it gives proper results even in the case where we just have a lecture and not an active term set.

[fosterfarrell9]

I added unit tests for lecture#stale?, made lecture#older_than? private (it is not used anywhere except by the lecture#stale? method) and added a cypress test for the lecture edit page that should fail when the page throws an error.

[Splines]

LGTM. Note that you'll have to approve the tests such that they can run through before merging via Squash and merge.

[codecov]

Codecov Report

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Additional details and impacted files

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  Files          ?      157           
  Lines          ?     6645           
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  Hits           ?     3579           
  Misses         ?     3066           
  Partials       ?        0           

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