extracted logic for activity completion

[yubrandon]

Isolated the logic for reusability

[DeMentidDev]

While I am unsure how I should be testing your logic, the website runs without errors. While everything is looking good so far, I won't be able to reliably merge your changes until I know how to test your logic.

(If the pull request isn't ready for a full review yet, then disregard this comment)

[yubrandon]

Most of the changes are in the backend, so I had to set static arguments for testing the requests made through completeActivity and manually check the database in order to test my changes.

I think that we would need to create a mock db to automate tests for this PR as well as other middleware implementations, but I am not familiar with how we would handle the methods being used since they were written for MongoDB.