Solves the build failure we're seeing on PRs

Fixes #18044
Related to #17416

Co-authored-by: NTim Neutkens <tim@timneutkens.nl>

Co-authored-by: NTim Neutkens <timneutkens@me.com>
Co-authored-by: Nkodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

Co-authored-by: NTim Neutkens <tim@timneutkens.nl>

Implements the `unsized` property from RFC #16832

cc @atcastle 

This makes sure the `locales` are passed to `getStaticPaths` and also disables the removing the default locale from the path when the default locale is the preferred header. It also updates tests to ensure the domain redirects are working as expected.  

x-ref: https://github.com/vercel/next.js/pull/17370

This closes [#18023](https://github.com/vercel/next.js/issues/18023). 

I was following the example mentioned in the issue (https://github.com/vercel/next.js/tree/canary/examples/with-firebase-authentication) and for some reason `xa` was always false, failing fetching food. Through `console.log` I saw that `ya` contained a token value and using `ya` worked. 

Bump resolve-url-loader version to fix vulnerability. 

Fixes #18048
Related #18044 

Co-authored-by: NTim Neutkens <timneutkens@me.com>

Co-authored-by: NTim Neutkens <timneutkens@me.com>

This makes sure that we detect the correct default locale for domain specific locales since a domain can have a different default locale residing at the root and we need to check this on the client for prerendered/auto-static pages. This also makes sure we disable the built-in redirect handling when on Vercel since it's handled already. 

Tests for this are tricky since we need to load the browser with a custom domain which requires editing the host file. Existing tests should ensure this doesn't break non-domain specific locale behavior though. This was also tested manually while testing https://github.com/vercel/vercel/pull/5298



x-ref: https://github.com/vercel/next.js/pull/17370

This updates the new image optimizer endpoint to instead of relying on the `host` and `proto` headers for relative files to use the internal route handling in `next-server` to load files from the public directory. The existing tests for relative files with the endpoint should cover these changes

Co-authored-by: NTim Neutkens <timneutkens@me.com>
Co-authored-by: Nkodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

Co-authored-by: NRémi de Juvigny <remi@hey.com>
Co-authored-by: NTim Neutkens <tim@timneutkens.nl>
Co-authored-by: Nkodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

Noticed that most of our mentions to Data Fetching are capitalized in both words, but the page itself wasn't. And it's not consistent with the titles in other sections

The current dependencies of  the example "with-react-native-web" are outdated.
In this PR I updated them and tested that they run and work.

Newest current react and react-dom versions: 16.13.1
Newest current  react-native-web and babel-plugin-react-native-web versions: 0.13.14

Mainly for the people starting with this example to have the new versions since the current versions are very far from each other.
Please merge 🙏 

Follow-up #17916

Correcting type errors and organizing redundant descriptions.

This makes sure the image optimizer doesn't de-animate images by transforming them with sharp since sharp doesn't currently handle outputting animated images

x-ref: https://github.com/vercel/next.js/pull/17749

Co-authored-by: NTim Neutkens <timneutkens@me.com>
Co-authored-by: NTim Neutkens <tim@timneutkens.nl>

This PR fixes 2 issues with the mongodb example:

### 1. Fallable Caching Strategy

Calling `connectToDatabase()` multiple times before it's cached results in multiple connections being created. The latest one created was becoming the "cached" one and the others dissappear into the background.

This is now fixed by using **promise sharing** so that only one connection can ever be created.

### 2. Problematic Hot Reload

During development you can monitor your database connections and see that it continues to create more and more connections over time. Some users have reported their [databases reaching maximum connection limits](https://github.com/vercel/next.js/discussions/12229).

This is resolved by using `global` to store the cached connection. It's not ideal but it is necessary.

* Stabilize i18n fallback test

* Handle reload during hydration check

![Screenshot on VS Code](https://user-images.githubusercontent.com/12539/96300300-43fc1280-1030-11eb-89ae-dba8beeca583.png)

The only attribute that must be used with Image Component is src, but all attributes are required and TypeScript will warn you.

Unify the confusing `config.images.sizes` ([Image Optimizer](https://github.com/vercel/next.js/discussions/17141)) and `config.images.breakpoints` ([Image Component](https://github.com/vercel/next.js/discussions/16832)).

Image Optimizer only accepts the name of the width parameter as a `w`.

ref https://github.com/vercel/next.js/blob/v9.5.6-canary.3/packages/next/next-server/server/image-optimizer.ts#L71