Make university selection a drop-down, rather than a free-form text field.

[SaltyQuetzals]

Is your feature request related to a problem? Please describe.
TAMUhack 2020 had freeform text fields so that students could enter their school instead of having to choose from a dropdown.

However, this made data analysis difficult.

Describe the solution you'd like
We should use a School model, and allow users to relate their Application to one of the School instances. Similar to the method mentioned in #339, we should create a fixtures file for our School model, so that users can simply load the fixtures up and have a list of schools auto-populated.

For users whose schools are not listed, we should include an "Other" School, and if selected, have them provide more details.

Describe alternatives you've considered
Using a MultiSelectField is less sustainable than a ForeignKey relation. Both will require a long list of schools as a file, so we might as well use something that Django supports out of the box.