Use flexbox for post settings cog & publish button

Closes #4208

- Removes `min-width` & `max-width` from `.publish-bar-actions`
- Uses flexbox to correctly handle dynamic size (due to font size and text content of button)

**Min font size set at 6** (en-gb/en-us default)
![screen shot 2014-10-12 at 19 28 18](https://cloud.githubusercontent.com/assets/390392/4607049/1a7b459a-523e-11e4-9ccd-12e3490ef3f7.png)

**Min font size set at 12** (zh-cn default)
![screen shot 2014-10-12 at 19 28 29](https://cloud.githubusercontent.com/assets/390392/4607052/23ac65ae-523e-11e4-90e9-a77b415ca2a1.png)

This also correctly fixes the broken layout when zooming in and out.