Switch from jQuery Ajax to Vue on issue page

What does this MR do?

• Replace all jQuery Ajax calls in Issue by Vue Resource calls.
• Add fallback for Ajax calls not mocked in tests.
• Convert spec_helper.js to ES6.
• Add missing error message for updating task list.

Why was this MR needed?

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

What are the relevant issue numbers?

fixes part of #24011 (moved)