Adding extractors isn't intuitive for new users

## Expected Behavior

A new user should be able to easily import extractor JSON without having to hunt around in the UI to find out where to add them.

## Current Behavior

See https://community.graylog.org/t/could-not-import-content-pack-invalid-json/21044. Currently it's not exactly an intuitive process for a brand new user to import extractors.

## Possible Solution

Provide a first-class menu item in the "System" tab as a way to add extractors.