Auto install bundle

Automatically install extensions "bundled" with installer on startup.

**Issues**:
* Should appReady in ExtensionManger be used instead?
* Terminology: folder name and location

cc @dangoor @peterflynn 

This is for #9233 
Not so. `FileUtils.getDirectoryPath` does basically what your code does: it chops off until the last slash. Since `getNativeBracketsDirectoryPath` is old-style and doesn't include the trailing slash, the result doesn't go into www.

```
> FileUtils.getDirectoryPath(FileUtils.getNativeBracketsDirectoryPath())
"/Applications/Brackets.app/Contents/dev/"
```

I agree with Peter's points also about `auto-install-extensions` and using a constant.