Pass individual sections to the post-processing function instead of the
entire list of sections. The only current use of post-processing is in
the `GlEink` screen type, which only supports one section, so there is
no requirement to be able to access all sections at once.

* Update departures widget generation to pass layout parameters from
  configuration to the frontend.

* Replace existing auto-sizing implementation, which only worked for a
  single section, with a version that handles multiple sections using
  the layout parameters.

* The auto-sizing itself (independent of the React component) is
  unit-tested. These are the first real frontend tests in Screens, so
  we take the opportunity to:

  * Update Jest to its latest version.

  * Replace the third-party `@types/jest` package with a direct
    dependency on `@jest/globals`, which is already part of Jest. The
    only advantage of the former is that it allows using `describe`,
    `test`, etc. without importing them.

  * Add `eslint-plugin-jest` to our ESLint setup.

  * Add `fishery` for defining factories to build test data.