Optimise css head

Shave 10KB (5KB gzipped) off of the CSS in `<head>`.

As a result, global.min.css is bigger (but we still have an overhaul gain).

## Before

```
Metrics for common/app/assets/stylesheets/global.min.css
>> Total rules: 564
>> Total selectors: 877
>> Average selectors per rule: 1.6
>> File size: 410.05 Kb
>> GZip size: 47.74 Kb

Metrics for common/app/assets/stylesheets/head.min.css
>> Total rules: 381
>> Total selectors: 613
>> Average selectors per rule: 1.6
>> File size: 40.47 Kb
>> GZip size: 11.16 Kb
```

## After

```
Metrics for common/app/assets/stylesheets/global.min.css
>> Total rules: 574
>> Total selectors: 858
>> Average selectors per rule: 1.5
>> File size: 415.49 Kb
>> GZip size: 52.11 Kb

Metrics for common/app/assets/stylesheets/head.min.css
>> Total rules: 355
>> Total selectors: 502
>> Average selectors per rule: 1.4
>> File size: 30.09 Kb
>> GZip size: 6.35 Kb
```

![Chewie](http://i.imgur.com/sP4IoVe.gif)