Include typechecking for all files in the CI

[lomky]

Changes in this PR!

• Adds the typescript build to our CI, so that all files are typecheckd
• Adds the missing types for jest with the @types/jest plugin
• Corrects all ~100 type errors in our tests. Types of corrections include:
  • Actual type fixes
    • see return type in Appointment.test.tsx as an example
    • see proper parameters in Header.test.tsx as an example
  • More explicit types, to help the TS compiler
    • see the types for objects passed to describe.each loops
  • Adding ? to checks where the base object could be undefined
    • Many places. i.e. element?.tagName.toLowerCase() === 'strong' in TransLine.test.tsx
  • Added type narrowing to assert the object is not undefined for assertions to use
    • See apiGatewayStub.test.ts if it does return undefined it should throw a failure
  • Explicit as unknown, as string overrides to test unexpected, mistyped values
    • See getScenarioContent.test.ts as example
  • Explicitly casts IncomingMessage in queryApiGateway.test.ts, to avoid a huge mock
• Adds two TS overrides for now, for places that may involve code changes. Will file tickets for these.
  • tests/pages/indexServerSide.test.ts - nextJS & typescript cannot agree on typing.
  • tests/utils/apiGatewayStub.test.ts - mocking for fs, fetch & typescript do not agree on typing
• Moves the getUrl.test.ts to a matching subdir to its utility

TK: tickets for resolving overrides:

• mocks + typescript (jest & fs) #587
• serverSideRender + typescript #586

===

Resolves #444

• [ ] Tests that fall out of date will alert the dev