Enhancement: Inject customizations

We have added the possibility to include own header scripts and styles.
This function is currently still experimental and there is a possibility that the api will change.

For the reasons mentioned, the functionality is not yet documented in the official documentation, but can be used as follows:

* to inject custom css add the following property to your `config.json`, `"styles": [{ "href": "css/custom.css", }]`.
* to inject custom scripts add the following property to your `config.json`, `"scripts": [{ "src": "js/custom.js", "async": true, }]`.

https://github.com/owncloud/web/pull/8432
https://github.com/owncloud/web/pull/7689
https://github.com/owncloud/web/issues/4735
