Update kses safecss_filter_attr function to allow gradient backgrounds

[jorgefilipecosta]

Currently, our KSES rules remove gradient backgrounds from style attributes (on contributor roles).
The block editor is introducing a mechanism to allow users to create and use a custom background. If our KSES rules are not updated, only roles with unfiltered HTML permissions will take advantage of this functionality.
I think our KSES rules should be updated. URL backgrounds, for example, are accepted, given that's the case, I think gradients should also be accepted. Ultimately what one can do with gradient background one can do with a URL background.

Related block editor issue:
​https://github.com/WordPress/gutenberg/issues/17938

Related block editor PR's:
​https://github.com/WordPress/gutenberg/pull/17603
​https://github.com/WordPress/gutenberg/pull/18001

[jorgefilipecosta]

As a test case, I did the following:
Logged as a contributor rule.
Opened code editor mode.
Pasted the following content:

<!-- wp:html -->
<div style="width: 100px;height: 100px;background: linear-gradient(red,yellow)"></div>
<!-- /wp:html -->

Submitted the post for review.
Reloaded the editor and verified the gradient was still being applied (without this patch, the gradient is removed).

GitHub PR where we can see the green CI light: ​https://github.com/WordPress/wordpress-develop/pull/110

[SergeyBiryukov]

In 46793:

KSES: Add support for gradient backgrounds.

Props jorgefilipecosta.
Fixes #48376.

[SergeyBiryukov]

In 46794:

KSES: Add support for gradient backgrounds.

Props jorgefilipecosta.
Merges [46793] to the 5.3 branch.
Fixes #48376.