When Staff creates a new Student User, their Profile is also created and its Cohort is dynamically populated

[halfghaninne]

Enhancement of feature in #92, which must be completed first.

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• [ ] Staff User goest to create new Student User, filling out name, github username, and email
• [ ] They are told that the student will be assigned to the cohort that hasn't yet occurred
  • [ ] If multiple cohorts are upcoming, they will be invited to select one
  • [ ] There is an option to click a link that says "select another cohort" and be given a drop down of all existing cohorts