Use border, not outline, for left nav

Using `outline` causes some of the `li`s to appear with a double border. This commit replaces the `outline` with a `border-top` and `border-bottom` and uses `margin-top: -1px` to collapse the two on all but the first `li`.

I had initially tried adding the `border-top` only for the first `li`, but that meant that when the inner levels were expanded, there was no border between the nth inner content and the n+1st main nav.

### Before:

![ember website nav - before](https://f.cloud.github.com/assets/2406/1063507/a1752afe-12c3-11e3-8be8-35bbbf4f497b.png)

### After:

![ember website nav - after](https://f.cloud.github.com/assets/2406/1063506/a164e6e4-12c3-11e3-9e19-3ae60289404f.png)