fix: improve PayPal popup handling

This commit includes modifications in PaymentDisplay.css and PaymentDisplay.js related to the PayPal recurring payment display.

In PaymentDisplay.css:

• Customizing the PayPal button hover effect using CSS.
• Changing button properties for a better responsive design.

In PaymentDisplay.js:

• Updating deprecated var declarations to const and converting to arrow function for clean and efficient code.
• Adding better error handling for failed PayPal window opening.
• Reformatting existing event listeners and function calls for code readability.

Updating the locale.xml to include error messages related to pop-up blocking in both German and English.

merged

mentioned in commit defb002c

This MR is included in version 2.1.1

The release is available on GitLab release.

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