Difficult to add mongodb lookup entry key values with spaces

When manually entering key value pairs in a MongoDB lookup the interface doesn't want to let you use spaces.

## Expected Behavior

You should be able to use space characters in keys. If you load key values with spaces through the API there are no issues.

## Current Behavior

The key values text field won't allow you to add spaces unless you type out all of the characters then go back and insert the spaces.

## Possible Solution

## Steps to Reproduce (for bugs)

1. Create a MongoDB lookup adapter
2. Go to "Create/update entries" and attempt to add a key value with spaces

## Context

## Your Environment

* Graylog Version: 3.3.0
* Elasticsearch Version: 6.8.9
* MongoDB Version: 4.0.18
* Operating System: Ubuntu 18.04
* Browser version: Firefox 76.0.1, Chromium 83.0.4103.61