Bug 1019841 - [Gallery] Update to use <gaia-header>

You're probably right. I was initially trying to keep the naming abstracted from Gaia. I've noticed that 'Font Awesome' (Github's open-source icon-font), uses the pattern:

```html
<div class="fa fa-icon-name></div>
```
Perhaps we could do something similar

```html
<div class="gi gi-icon-name></div>
```