Add loading icon component

[JosephusPaye]

Adds an an animated loading icon. Extracted from #234.

Stories:

https://nusight-pr-236.herokuapp.com/storybook/?selectedKind=components.loading_icon&selectedStory=renders%20animated&full=0&addons=1&stories=1&panelRight=0&addonPanel=storybook%2Factions%2Factions-panel

[BrendanAnnable]

I had a bunch of tiny feedback items that would take a while to communicate via text, so I've just submitted a commit. It does:

• Remove the div wrapper.
• Simplify the styles to avoid relative/absolute positioning.
• Set the width/height on the SVG itself.
• Make the component a PureComponent instead of an observer (since it doesn't take any observable values)
• Remove the classname prop, and inline the props type
• Rename style.css to styles.css (and its local import variable)

If that LGTY then this PR LGTM 😄

[JosephusPaye]

LGTM, thanks!