What's new popup

[peterzimon]

ref DES-192

We often hear that people are not aware of the new features we ship. Ways in which people can find out are social media/changelog/dashboard – all of these are easy to miss. We'd like to introduce a template for a simple popup that can be used any time a new and noteworthy feature has shipped. The purpose of this popup is simply to notify and will disappear forever after it's been dismissed.

In this PR I added a modal that appears when there's an unseen featured article in Changelog ("unseen" rules remained the same).