KSES: Allow `url()` to be used in inline CSS.

The cover image block uses the `url()` function in its inline CSS, to show the cover image. KSES didn't allow this, causing the block to not save correctly for Author and Contributor users. As KSES does already check each attribute name against an allowed list, we're able to add an extra check for certain attributes to be able to use the `url()` function, too.

Merges [43781] from the 5.0 branch to core.

Props peterwilsoncc, azaozz, pento, dd32.
Fixes #45067.


Built from https://develop.svn.wordpress.org/trunk@44136


git-svn-id: https://core.svn.wordpress.org/trunk@43966 1a063a9b-81f0-0310-95a4-ce76da25c4cd