Added visual effects to ember settings admin.

Closes #2749
- Created `settings-pane` view to `fadeIn(300)` on didInsertElement
- Apps, debug, general, and user settings views all extend settings-pane
- Added link-views to settings template to activate correct tabs.

This works, but I'd be happy to have feedback on two things (well, and anything else, of course)
1) Is it not possible to just say somewhere "All the settings routes use the same view" rather than having four identical files?
2) Is it necessary to call extend on the settings-pane view when we're exporting it for each of the different views?
ie, from this:
```javascript
import settings-pane ....;

export default settings-pane.extend()
```
to this:
```javascript
export default settings-pane
```