The GitLab UI update changed the spinner selector from `spinner` to
`gl-spinner`, so we need to update it, so that our tests won't fail.

**Why?**
This file only contains a few text based helper functions.
It has nothing to do with Vue or Vue components.

A number of the karma tests looked for the .fa-spinner class or the
icon element used for the font awesome spinner class.

These instances have been refactored to now look for the .spinner
class instead.

Instead of rendering all report items in 4 big lists, we make use of
vue-virtual-scroll-list and render only few dozens at once. This
improves the performance in several metrics:

- Initial load time
- Memory Pressure
- CPU Load
- DOM node count

In an example with around 11k reported security vulnerabilities:

- Initial load time: 27s -> 4.1s
- Memory Pressure: ~750 MB -> ~270 MB
- CPU Load (time spent on executing JS/Rendering): 22s -> 2.5s
- DOM node count: 430k -> 7k up to 30k while scrolling