* Moved server/lib/utils.js to Typescript
* moved _app.js to Typescript
* Moved _error.js to Typescript
* Added argument for custom props in _app and _error
* Moved _document.js to Typescript
* updated one test
* Updated types and added a validation for _document props
* Improved types
* Fixed some types
* Updated AppType
* Fixed some tests
* Added missing import
* Removed a not very useful type
* Fix missing type
* Move @types/styled-jsx
* Fix typescript errors
* Throw error if getInitialProps is defined as as instance method
Omitting the static keyword happens pretty often. Therefore we should trigger a warning in devmode.
Closes: #4782
* Document getInitialProps error
* Add unit tests for loadGetInitialProps