Creating extractor from an existing message always uses the input id from the sample

## Expected Behavior
Creating an extractor on a specific input should associate the extractor with that input, not the input the sample message was received on.

## Current Behavior
Due to https://github.com/Graylog2/graylog2-server/blob/master/graylog2-web-interface/src/components/search/MessageFieldExtractorActions.jsx#L26 the add extractor workflow always takes the input id the sample message has been received on, instead of the explicitly picked input (when going through System/Inputs -> Manage Extractors).

## Possible Solution
Pass in the explicit input id from context when creating the add extractor form.

## Steps to Reproduce (for bugs)
1. Create two inputs.
2. Send message to input 1 and not the message id and index.
3. Go to System/Inputs -> Input 2 -> Manage Extractors
4. Load sample message via id/input
5. configure any extractor and save it
6. the extractor will be created on input 1, not on input 2.

## Your Environment

* Graylog Version: 3.3.5 (but present since at least 2.0)