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 |
|:------:|:-----:|
| ![Screen_Shot_2015-04-23_at_4.14.23_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/c77cb0816bf0f05cb8c4105cdd8b3ec6/Screen_Shot_2015-04-23_at_4.14.23_PM.png) | ![Screen_Shot_2015-04-23_at_4.15.44_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/463c1d2052e12444563f5e2c102ac51e/Screen_Shot_2015-04-23_at_4.15.44_PM.png) |

### 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.

See merge request !569