Drag & Drop upload/parsing

## What?

A super awesome ™️ feature to have would be drag & drop upload and parsing of files. Say for example that I actually want to examine my Graylog, MongoDB or Elasticsearch logs, rather than having to set up filebeat and some special pipelines, I could just drag and drop them into the UI where they'd be automatically parsed. This would save time on going through the exercise of configuring filebeat or some other log ingestion agent just to upload a singular log file.

## Why?

Configuring filebeat or another ingestion agent is heavy-handed for this sort of task and this seems like it would be a quick and easy thing to do to make the experience of using Graylog a bit less cumbersome.