* Set default `Error` status code to 404

This is an appropriate default behavior because:

1. When the server encounters an error, the `err` property is set.
2. When the client-side application crashes, the `err` property is set.

This means the "only" way to render the `/_error` page without an error
is when a page is not found (special condition).

Fixes #6243
Closes #5437

* Add new integration test for client side 404

* single quotes

* Remove unused variable

* Standard needs to go away

* Whoops

* Check for null status code in res and err

* Only check response for valid statusCode

* Add `module` as server fallback main field

* Test that a module only package can be imported

* convert export default anonymous functions into named functions

* change examples to function declaration and split export in classes

* change NextHead name to Head and rename component

`initialProps.styles` is a React node, but not guaranteed to be an
array, so we can use a fragment to concatenate additional styles.

See https://github.com/DefinitelyTyped/DefinitelyTyped/issues/32932#issuecomment-462372319

* extract request login from auth

* add clarification that the monorepo is for deploy in Now only and fix typo

* Refactor HOC

- add authorization to HOC
- add displayName to HOC
- remove unnecessary `run`s in local routing

* Updated examples for build-time env configuration for v8

* Add comment to build time config example with how to include entire .env

As per discussion in #6165, I removed the `passHref` in `Link` as
they're unnecessary.

Changelog https://github.com/zeit/styled-jsx/releases/tag/v3.2.1

* Bring in terser-webpack-plugin

* Ignore terser from linting

Introduce env to next.config.js for build-time configuration

AFAIK this dependency isn't being used anywhere so it should be safe to remove it.

This updates the react-jss example to work with the v10 alpha

Followup to removing the /_error in the initial markup we can now compile it using on-demand-entries only when it's needed.

`Router.events.off` should be called instead of `Router.events.on` in `componentWillInmount`.

Fixes #5860

Closes #6152.

- Only dynamically load /_error.js when an exception occurs.
- Fix websocket “/_error.js” ping oddity.

I've just thought of a way to improve the initial props page by adding
an example for a list/detail page structure. To do that, I've created a
separate `/detail` page, and a mock API which calls data from the array
we made on the previous PR.

A ListDetail component is created as an example for displaying detail.

Page structure is also cleaned up. Should I go ahead and add an example
on how to style with styled-jsx + its TS declarations? I might decide to
do it within this week anyway.

Update examples with-relay-modern and with-relay-modern-server-express to react-relay 2.0.0

- react-relay has started to use new Context API instead of Legacy Context API
- add `parseInt` because graphql 14.0.0 introduced stricter scalar value coercion

Closes #6157

This reverts the changes made in [this pr](https://github.com/zeit/next.js/pull/6109).
`redux-saga: "1.0.0"` changed the way it handles it's queues. Because of that we're still having trouble to implement the synchronous side-effects flow in `next-redux-saga`. See [this discussion](https://github.com/bbortt/next-redux-saga/pull/1) for more information.
Therefore I would feel more comfortable not to mislead users by giving them a non-working example in the main branch.