Use dashicons-class in `wp_dashboard_quota` and `wp_dashboard_right_now`

[kebbet]

Proposal:
Replace hardcoded css-rules for each item in the At a Glance-widget, use dashicons-classes for each item. This makes adding new items to the widget easier, simply just add a dashicons-before-class the your new items.

Background:
So using dashboard_glance_items to add custom items to the post list is easy, but changing icons without adding new css with a file or inline to the admin views is hard, since the hardcoded rules overrides dashicons-before on my added items.

https://core.trac.wordpress.org/ticket/55521

[kebbet]

So now when using dashboard_glance_items I can add icons to items with <a class="dashicons-before dashicons-carrot" href="?edit"> instead of adding custom CSS-files or inline CSS to my plugin.

[kebbet]

Patch is updated against trunk.