feat: add Explore area

Created by: sqs

The Explore area shows information and actions from various product features. It is intended to make users aware of more features and show useful information at a glance.

In this commit, it is hidden behind the feature flag localStorage.explore=true;location.reload().

Designs: https://app.zeplin.io/project/5bd785e9585e2b0a696c976e/screen/5bd786510cd71b0a7b2c7d96

Screenshot: