Astra <= 4.12.3 - Authenticated (Contributor+) Stored Cross-Site Scripting via Post Meta

The Astra theme for WordPress is vulnerable to Stored Cross-Site Scripting via the `ast-page-background-meta` and `ast-content-background-meta` post meta fields in all versions up to, and including, 4.12.3. This is due to insufficient input sanitization on meta registration and missing output escaping in the `astra_get_responsive_background_obj()` function for four CSS-context sub-properties (`background-color`, `background-image`, `overlay-color`, `overlay-gradient`). This makes it possible for authenticated attackers, with Contributor-level access and above, to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page.