fix: date label for instructor paced courses

[Anas12091101]

Description

In the instructor paced courses, in which a user enrolls after the end date, there's a bug of showing label of "Enrollment Date" instead of "Start date" on the important dates page in LMS. This PR solves this issue by matching the conditions in date and title property of CourseStartDate class.

Useful information to include:

• Which edX user roles will this change impact? Common user roles are "Learner"

Supporting information

https://github.com/mitodl/hq/issues/5889

Testing instructions

• On the master branch, create an instructor paced course and set the start and end date to a past date in studio.

• Enroll in the course from a test learner account and open the course in LMS
• Click the Dates tab. You will notice that the date label is 'Enrollment Date', and the displayed date corresponds to the start date.

• Now, checkout to this branch and reload the above dates page. It will show the correct label.

Deadline

"None"

Other information

Include anything else that will help reviewers and consumers understand the change.

• Does this change depend on other changes elsewhere?
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• If your database migration can't be rolled back easily.

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Thanks for the pull request, @Anas12091101!

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