Membership Access Plan allows Student Buy an Expired Membership.

### Reproduction Steps

+ I created an Access Plan for a Membership.
+ Expiration Date for two weeks in the past (04/01/2019).

### Expected Behavior

+ LIfterLMS would either warn the Admin or the Student that this Membership had already expired.

### Actual Behavior

+ Membership was published
+ Student was able to buy Membership and was enrolled in a course.
+ Student could enter the course and complete assignments.
+ As soon as I hit submit on an assignment it would bounce me out and remind me I had to "enroll in this course before I could take it"

### Error Messages / Logs

+ Include any relevant error messages or log files
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### System Information

System Report

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### Browser, Device, and Operating System Information

+ Chrome, MacOS