Index Set Configuration Dialog uses two different inputs for time durations/intervals.

## Expected Behavior

Entering time durations/intervals is consistent throughout the product.

## Current Behavior

When configuring an index set with a time-based rotation strategy, two durations/intervals need to be configured. One is the "Field type refresh interval", the other is the "Rotation period". The first one is configured with a value input and a unit dropdown:

The rotation period is configured using a plain input that expects the user to enter a ISO8601 definition for the duration, without providing the available time units:

Having two different ways to configure the same type (on the same page) and forcing users to wrap around their head ISO8601 durations when all they want is configure the rotation period is suboptimal.

## Possible Solution

The inputs should be unified so entering durations/intervals is consistent.

## Steps to Reproduce (for bugs)

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## Context

## Your Environment

* Graylog Version:
* Elasticsearch Version:
* MongoDB Version:
* Operating System:
* Browser version: