Fix search_fields in education app

Ghost Userrequested to merge
fix/education-search into master

Closes #359 (closed)

It's not possible to search within course and uploader because these fields are not strings.

Before:

  1. Search in the exam or summary model
  2. Get an icontains error

After:

  1. Search in the exam or summary model
  2. No errors, appropriate results

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