Rollup merge of #71237 - Cldfire:rustdoc-ayu-theme, r=GuilliaumeGomez

Add Ayu theme to rustdoc

This is a port of a theme I maintain (https://github.com/Cldfire/ayu-rs) to the native rustdoc theme system. [Ayu](https://github.com/dempfi/ayu) (dark) is a richly-colored dark theme that many people enjoy using across a wide variety of environments.

Corresponds to the Ayu theme in [mdBook](https://github.com/rust-lang/mdBook).

Some screenshots:

![image](https://user-images.githubusercontent.com/13814214/79547087-6c935780-8061-11ea-8a33-38e9472e9fec.png)

![image](https://user-images.githubusercontent.com/13814214/79547150-8339ae80-8061-11ea-97be-9e13a8b275d7.png)

![image](https://user-images.githubusercontent.com/13814214/79547221-98164200-8061-11ea-9649-9b11ccbb33e3.png)

![image](https://user-images.githubusercontent.com/13814214/79547310-b419e380-8061-11ea-9965-d4f90b2280ab.png)

![image](https://user-images.githubusercontent.com/13814214/79547443-e7f50900-8061-11ea-8872-06d74010691e.png)

Note that this pull request also makes some small code changes to allow for disabling theme stylesheets, preventing the rules from all the different themes from conflicting with one another. The only stylesheet that is not disabled is `light.css`; the theming system (quite hackily) switches themes by changing the href on this stylesheet and so permanently disabling all the others works perfectly fine.

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