Dashboard widget redesign

[hedgefield]

We want to redesign the widget on the wordpress dashboard. The OnPage section is to be removed, and the stats section gets a visual upgrade.

Colors:

$color_bad
$color_ok
$color_good
$color_score_icon

If these don't seem to match the colors in the mockup, that's right, those will be updated in the CSS later (via #440), so don't worry.

Ping me when you need the illustration.

Related (problem): #7130

[hedgefield]

Based on #7786 I added the two latest blogposts to this widget too.

[omarreiss]

I expect the frontend team to create React interfaces for both parts of the widget which can be implemented by the backend team.

[moorscode]

Please unblock #7818 after completing.

[abotteram]

Split up into Yoast/yoast-components#265, Yoast/yoast-components#266, Yoast/yoast-components#267, Yoast/yoast-components#268

This issue now encapsulates the implementation of the Dashboard widget, meaning that all issues mentioned above are combined into one component.

Note: edited by @afercia, the links to the split issues were pointing to this repository:
#165, #166, #167 and #168