Remove <h2> from flag dialog

Applies to https://github.com/GigaOM/gigaom/issues/5287

Changes `<h2>` to `<header>`

![screen shot 2014-10-20 at 5 29 03 pm](https://cloud.githubusercontent.com/assets/929375/4709597/8dcf0014-58a0-11e4-8b3f-f76958fd3073.png)

#### Requires Styles

https://github.com/GigaOM/gigaom/pull/5615