Auto merge of #7030 - xFrednet:6877-clippy-going-dark, r=llogiq

Clippy going dark: Adding a dark theme to Clippy's lint list

This PR adds the MdBook color themes to the lint list of Clippy. Well at least an adaption of these themes.

<details>
<summary>Here are some beautiful screenshots:</summary>

**light theme**
![image](https://user-images.githubusercontent.com/17087237/113510593-e31fb280-955b-11eb-8ab1-8b5bcf287475.png)

**Rust theme**
![image](https://user-images.githubusercontent.com/17087237/113510734-79ec6f00-955c-11eb-981c-8ebe890acf79.png)

**Coal theme**
![image](https://user-images.githubusercontent.com/17087237/113510752-8ec90280-955c-11eb-8f5c-c87ca07c35c2.png)

**Navy theme**
![image](https://user-images.githubusercontent.com/17087237/113510675-3f82d200-955c-11eb-8992-8c784abe19ea.png)

**Ayu theme**
![image](https://user-images.githubusercontent.com/17087237/113510700-588b8300-955c-11eb-83e0-a8f770e9f913.png)

</details>

The theme is also stored in the browser to ensure that the next session applies the theme and doesn't burn your eyes out.

cc: `@matthiaskrgr`

---

Closes #6877

changelog: [Clippy's lint list](https://rust-lang.github.io/rust-clippy/master/index.html) now supports themes

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