Add type safety and type hinting

[ashleyzhang01]

Type safety is important for catching bugs early, facilitating refactoring, and improving overall maintainability.
We should use type hints makes the code more understandable to both developers and static type checkers, so our code can be more reliable and predictable

We should:

• Use type hints
• Write code in a more functional style
• Add comments for things like magic values, unintuitive behaviors, gotchas, external context, etc.
• Types should have capitalized names
• Keep things pythonic (e.g. use list comprehensions, avoid explicit for-loops, use built-in functions and libraries)
• Use type aliases to give semantic meaning to primitive types (e.g., ModelPath = str)
• Functions should typically return a singular type rather than unions
• Functions not used outside their module should be prefixed with _
• Use context managers (with statements) for resource management
• Consider using Protocol classes for structural typing
• Use assert to help type checking when function logic limits return types
• Use typing.cast as a no-op to inform the type checker of a type when needed
• Use schemas instead of args and kwargs 😭

TODO: (in the works) add mypy for type checking

[codecov]

Codecov Report

Attention: Patch coverage is 93.44262% with 28 lines in your changes missing coverage. Please review.

Project coverage is 90.68%. Comparing base (96a605c) to head (848c72b).

Files with missing lines	Patch %	Lines
backend/sublet/views.py	83.87%	10 Missing ⚠️
backend/sublet/serializers.py	91.22%	5 Missing ⚠️
backend/gsr_booking/views.py	94.00%	3 Missing ⚠️
backend/gsr_booking/api_wrapper.py	93.75%	2 Missing ⚠️
backend/gsr_booking/models.py	84.61%	2 Missing ⚠️
backend/gsr_booking/serializers.py	81.81%	2 Missing ⚠️
backend/portal/logic.py	81.81%	2 Missing ⚠️
backend/user/views.py	85.71%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #317      +/-   ##
==========================================
- Coverage   90.86%   90.68%   -0.19%     
==========================================
  Files          63       63              
  Lines        2661     2779     +118     
==========================================
+ Hits         2418     2520     +102     
- Misses        243      259      +16     

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[Clue88]