Minor design change grab-bag 🎉

I started with one simple change and found a few more, so here they are.

Simplify icon style selectors

All FontAwesome icons have an fa class, so just use that.

Don't override color of code blocks in notes

This was a pet peeve of mine.

Before	After

Items with tooltips don't need a 'data-original-title' attribute

This attribute gets added dynamically by Bootstrap's tooltip JS based on the title attribute

Fix Profile > Design live-updating

ui_blue wasn't added to the list of classes to remove, so if a user changed to that theme, any subsequent changes wouldn't be live-updated.

This change refactors Gitlab::Theme a bit to make it harder for this to happen in the future with new themes.

Remove the has_bottom_tooltip class

Bootstrap's tooltip JS can read the placement from a data-placement attribute.

Further, when we supply the selector option to tooltip, tooltips will be added to any dynamically-added elements matching the selector, without us having to re-call the tooltip method.