Globally configure start page for users

## What?

As a Graylog operator/administrator, I'd like the ability to globally configure a default stream for users.

## Why?

In a large environment where the default search page must load quite a lot of data, setting a default start page where the amount of data returned is constrained would help provide my end users with an experience where they're not waiting a long time on the page to load.

## Your Environment

See HS-472485922 for details.