Add edit/command mode indicator

Fixes #4889

Here is the current look:

![screen shot 2014-02-05 at 10 59 19 pm](https://f.cloud.github.com/assets/27600/2096225/4853bd04-8efc-11e3-9990-5f35090842bf.png)

I couldn't use the checkbox'y icon pointed out in the above issue as the two were different sizes. There was also talk about adding an `edit_mode` and `command_mode` CSS class so some global element, but I am unconvinced this is needed and don't even know which element to attach it to.