Fix Mypy errors and small fixes

[jpmckinney]

Migration is safe: SELECT * FROM application WHERE award_id IS NULL OR borrower_id IS NULL; has 0 rows.

• fix: Refresh signing key if Cognito rotated it
• chore: Fix simple mypy errors in Cognito client usage
• chore: Split ResetPassword from BasicUser Improve implementation of "multiple models" #376
• chore: Ignore missing type info that can't be fixed without changing third-party module
• fix: Make Application.award_id and borrower_id non-nullable
• chore: Align model with model_id nullability, except Application.lender non-nullable (like .credit_product since b3d05bf)

[coveralls]

Pull Request Test Coverage Report for Build 10540159434

Details

• 21 of 29 (72.41%) changed or added relevant lines in 5 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.1%) to 85.984%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
app/aws.py	0	1	0.0%
app/auth.py	9	11	81.82%
app/routers/users.py	0	5	0.0%

Files with Coverage Reduction	New Missed Lines	%
app/aws.py	1	63.64%
app/routers/statistics.py	2	67.57%

Totals
Change from base Build 10540122049:	-0.1%
Covered Lines:	1822
Relevant Lines:	2119

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💛 - Coveralls

[jpmckinney]

@yolile Merge whenever you're ready. There is a migration (but it should not affect the app).