feat: github-hosted presets (#2328)

Adds support for GitHub-hosted presets, using the `github>` prefix. Supports single preset per repository only.

Example:

```json
{
  "extends": ["github>renovatebot/renovate"]
}
```

The above would extend the `renovate.json` inside this repository (probably not a good idea, but an example).

Closes #2102, Closes #2312