Code Editor: Extend CSS linter to check for safecss-forbidden properties

[westonruter]

While there is a custom HTMLHint rule which checks for Kses violations, this does not extend to CSSLint and safecss_filter_attr(). It would be a nice enhancement to get illegal style properties flagged as lint errors as well.

Originally ​https://github.com/WordPress/better-code-editing/issues/60:

The safecss_filter_attr() function is used by KSES to filter HTML style attributes, removing any properties that are illegal. This should be applied to CSS and style attributes in HTML, so that any illegal properties are flagged as such.

It seems that HTMLHint is not currently applying CSSLint to style attributes, but if it did, then a kses rule added to CSSLint could then apply to both HTML and CSS.

This is for the code editor introduced in [41376] for #12423.

[westonruter]

Bumping priority to high for visibility and alignment with 4.9 goals, and given proximity to beta 1 deadline.