fix(Session): Remove Session ID from Redis after logout

[hirokiterashima]

Test

• Login as teacher or user, and check Redis keys keys *. You should see an entry like this:
  spring:session:index:org.springframework.session.FindByIndexNameSessionRepository.PRINCIPAL_NAME_INDEX_NAME:username
• Logout as the user and check Redis keys again. The entry should be deleted. Before, this entry was not getting deleted.

Closes #167

[geoffreykwan]

Looks good.

I think we might need to run flushall after deploying this update.

• I previously had the key
  "spring:session:index:org.springframework.session.FindByIndexNameSessionRepository.PRINCIPAL_NAME_INDEX_NAME:GK"
  in my local Redis
• I applied the pull request change
• I signed in as GK and then signed out
• The key was still in my local Redis after I signed out
• I ran flushall
• I signed in as GK and it recreated the key
  "spring:session:index:org.springframework.session.FindByIndexNameSessionRepository.PRINCIPAL_NAME_INDEX_NAME:GK"
• I signed out from GK and the key was then removed

[geoffreykwan]

🎉 This PR is included in version 1.0.0 🎉

The release is available on GitHub release

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