Identify components to componentize

Let's figure out some components! CC @kwight

Initial ideas that come to mind that follow the standardish class structure pretty nicely:

```
|- masthead
|-- site-title

|- navigation

|- hentry // for post and pages?
|--- entry-title
|--- entry-meta // and maybe components for the stuff that lives here
|--- entry-content

|- comment-box
|--- comment-form
|--- comments
|----- comment
|------- author ingo/meta stuff?
|------- comment-content
|------- comment-actions // reply/favorite/etc

|- widget
```

The issue lists possible theme components but names no files, tests, or entry points. Start by locating the existing React theme structure and compare it with the proposed masthead, navigation, hentry, comment-box, and widget boundaries; done requires an agreed componentization plan.

Written by the indexing model from the issue text.