Tag descriptions

Both 'series content containers' and 'tag front containers' may have a description. Their appearance is similar, aside from variations in breakpoints.

This PR generalises the existing series description so it can be used by both tags and series. There should be no visible change to the series container, only changes to the tag page, which follows similar breakpoint behaviour (except there is no 'series' label).

**Before (tag page with an ignored description)**
![screen shot 2014-08-15 at 16 10 19](https://cloud.githubusercontent.com/assets/4549425/3934710/83446882-248e-11e4-82be-ea833eb842e8.png)

**After (wide breakpoint)**
![screen shot 2014-08-15 at 16 10 27](https://cloud.githubusercontent.com/assets/4549425/3934716/8d38a416-248e-11e4-916b-a507b08b3226.png)

**After (desktop breakpoint, known as half-width breakpoint in code)**
![screen shot 2014-08-15 at 16 10 58](https://cloud.githubusercontent.com/assets/4549425/3934739/d68d9ffe-248e-11e4-894a-b4b1ac8e45fc.png)

**After (tablet breakpoint, hidden on mobile)**
![screen shot 2014-08-15 at 16 14 42](https://cloud.githubusercontent.com/assets/4549425/3934765/0acaf208-248f-11e4-9560-0816d104ec62.png)

Another perilous friday shipping.

![megalodon_takes_a_bite_of_ship_wallpaper](https://cloud.githubusercontent.com/assets/4549425/3934778/36aa26b4-248f-11e4-8e75-0f1d3dc5a42c.jpg)