Don't install system extensions

The extension manager currently refuses to install extensions that are already installed the user's extension directory, but will happily install extensions that already exist in the system default extension directory. This can cause problems with extensions like Edge Web Fonts that are freely distributed online, but which are installed by default in Edge Code. This pull request adds an additional required option, `systemDirectory`, to the `install` command in the `ExtensionManagerDomain` that is used to avoid conflicts in the same way as the `destinationDirectory` parameter.