Remove EnrollmentComplete experiments from pool if the user is not already enrolled

[bcb37]

An edge case bug showed up where a student did not get an assigned condition for an enrolling experiment. This was due to the pooling logic choosing an "Enrollment Complete" experiment with the same decision point in the same context.

To Do:

• Change the pooling logic to remove experiments with "Enrollment Complete" status from pools that do not contain an experiment that the user (or user's group) is enrolled in.

[amurphy-cl]

Case example
Exp 1 with dp A and B starts 09/01, enrolls John Doe
Exp 1 is set to Enrollment Complete with Post Rule as Assign: Default on 09/30. Because the Post Rule is Assign:Default, this status should allow any participants previously enrolled in Exp 1 to be eligible for new experiments with shared decision points. Any other Post Rule status (Assign: [any condition not default] or Continue) blocks the participants from being assigned a condition in an experiment with a competing decision point.

Exp 2 with dp A and B starts 10/01 - John Doe should be able to be assigned a condition in this experiment since the Post Rule for Exp 1 was Assign: Default.