Create Index Set has Mandatory Fields but the UI does not imply so

To create an Index Set you have to fill out the mandatory fields. Until then the Create
Button is disabled. Unfortunately the UI does not show which fields are mandatory in any way.

## Expected Behavior
Mandatory fields should be visible so the user does not have to guess what fields he needs to fill out.

## Current Behavior

## Possible Solution
Either mark the fields as mandatory in any way or enable the create button so the user get
actual error messages which display the missing fields.

## Your Environment
* Graylog Version: Graylog 5.2.0-SNAPSHOT+edc69d6