z-index tweaks for #user-dropdown and align the #topic-admin-menu icon

- Minus #show-topic-admin button z-index by 1 which will be covered by #user-dropdown
- Add 14px width for #topic-admin-menu icon, align the button text

Before
![2](https://cloud.githubusercontent.com/assets/297343/2601795/e4f8171e-bb15-11e3-9067-7980ee495b95.png)
After
![1](https://cloud.githubusercontent.com/assets/297343/2601794/e4f6e6c8-bb15-11e3-867c-75a83f050504.png)
![3](https://cloud.githubusercontent.com/assets/297343/2601796/e4fdc574-bb15-11e3-9e98-1d7cc9ba7fb5.png)