(Re)Introduce the category improvements in the notification center

[benvaassen]

Currently, there are 2 categories: Problems and Notifications. There is no way to let customers know that some notifications are actually improvements (see the image). We should bring back the category "Improvements" to inform our customers better.

Image:

Technical info

• WordPress version: 4.7.3
• Yoast SEO version: 4.4

[jdevalk]

This needs better design :-)

[omarreiss]

This needs to be taken into account when redesigning the dashboard.

[omarreiss]

See https://github.com/Yoast/roadmap/issues/20