WPML String Translation 3.2.15

Added a small performance fix.

Fixed an issue with registering data attributes content as strings.
Fixed an issue with registering MO files for some languages.
Fixed the missing expiration time for an upgrade lock.
Added support for WordPress 5.8 widget blocks.
Removed legacy multilingual widgets and prepared their migration to text widgets.

Fixed issue with wp_localize_script in WordPress 5.7.
Fixed error on String Translation page when string contains encoded HTML entities.