Convert to function components & React Hooks

The code organization would be a lot clearer if we used [function components](https://reactjs.org/docs/components-and-props.html) and [React Hooks](https://reactjs.org/docs/hooks-intro.html) instead of class components.

We now have `useUserState` and `useUserStateActions` hooks to access the state of the app and manipulate it, respectively.

Converted [`src/pages/*`](https://github.com/codeforboston/windfall-elimination/tree/develop/src/pages) to function components?:
- [x] 404.tsx
- [x] About.tsx
- [x] faq.tsx
- [x] index.tsx
- [x] prescreen-1a.tsx (Background / birthdate entry screen) #294
- [x] prescreen-1b.tsx (Earnings screen) Thanks @sashamaryl #294
- [x] prescreen-1c.tsx (Employment Status /windfall elimination question & pension specificiation) #294
- [x] prescreen-1d.tsc (future earnings) - thanks @dylanesque #251
- [ ] print.tsx
- [ ] screen-2.tsx (results with both calculators)
- [x] screen-2a.tsx (Example walkthrough of calculations) - thanks @linesbetween #141

Other major components:
- [ ] [`src/components/file-upload.tsx`](https://github.com/codeforboston/windfall-elimination/blob/develop/src/components/file-upload.tsx) (can be separated into individual components)

Feel free to create new component files as you go.

Start by reviewing the remaining class components in src/pages/print.tsx and src/pages/screen-2.tsx, then inspect src/components/file-upload.tsx and the existing useUserState and useUserStateActions hooks. Convert the remaining components to function components using those hooks, including any separated file-upload components, and confirm no listed class components remain.

Written by the indexing model from the issue text.