wp-admin CSS audit

[isabel_brison]

This is an umbrella ticket for an audit of the CSS in wp-admin, as discussed in the #core-css chat: ​https://make.wordpress.org/core/2020/03/03/css-chat-summary-27th-february/.

The audit should cover the following aspects, for which separate tickets may be created if they don't exist already:

• Identifying where specificity can be reduced;
• Identifying dead code;
• Identifying code specific to browsers no longer supported (existing ticket: #46015);

There is a ticket for auditing the toolbar CSS: #46003.

There is also a ticket for auditing all occurrences of !important: #26350, and another one tackling just the !importants in `common.css: #47569.