Wrapper for HTTP requests / Ajax calls

What does this MR do?

• Introduce wrapper for HTTP requests / Ajax calls.
• Add Promise polyfill to JavaScript tests.
• Add fallback for Ajax calls not mocked in tests.
• Convert spec_helper.js to ES6.
• Replace all jQuery Ajax calls in Issue by Http wrapper.
• Add missing error message for updating task list.

Why was this MR needed?

We want to remove jQuery (#23022 (closed)).

What are the relevant issue numbers?

#24011 (moved)