Web interface should warn about extractors for non-string fields

## Expected Behavior

The web interface should not allow users to create extractors for non-string message fields as they won't be executed.
## Current Behavior

The web interface allows creating extractors for non-string fields, e. g. numeric fields common for HTTP response status codes.
## Possible Solution

The web interface should either not allow creating extractors for non-string fields or issue a big warning about it.
## Steps to Reproduce (for bugs)
1. Create a random input.
2. Create a "Copy Input" extractor for the `http_response_code` message field.
3. Wonder why the `http_response_code` isn't being copied.
## Your Environment
- Graylog Version: 0.20.x - 2.x