Improve external-content typographic styles

One more design request from Ben: improve the quality of typography for external-content/blocks of copy.

- Extracted out all opt-in typographic styles into `_from-content-api.scss`, matching next-gen. _We're not using the main Content API but it does serve the same purpose (content comes from Eventbrite API) so I thought it made sense to follow the same convention_
- Style lists to match next-gen style and to handle nested lists correctly
- Clean-out some now redundant styles from `_events-detail.scss`

**Pattern library example**
![screen shot 2014-12-30 at 17 23 52](https://cloud.githubusercontent.com/assets/123386/5580923/b8cd09b0-9049-11e4-9a3b-1df55215ba26.png)
![screen shot 2014-12-30 at 17 30 44](https://cloud.githubusercontent.com/assets/123386/5580913/a01757ea-9049-11e4-9866-a3fc16a5550f.png)

**Masterclass example**
![screen shot 2014-12-30 at 17 24 22](https://cloud.githubusercontent.com/assets/123386/5580891/548bd8be-9049-11e4-96a4-ae3eed2d4dc7.png)
Next-gen actually does this using a mixin and generated content rather than an image, but using the standard `list-style-image` allows for less overriding of default list styles and allows us to maintain `list-style-position`. Conceptually it's an image too.