Admin navigation pane

I'm opening this issue to continue the [discussion](https://groups.google.com/forum/#!topic/mezzanine-users/FigDQnoTlrs) from the group. This is what I've done so far:

1. Revert typeface to Arial.
1. Make the side pane scrollbar appear only when necessary.
1. Clean up the CSS.

Some other changes I would like to recommend:

1. Add a toggle to hide/show the pane? The agreed best approach is to use a thick border on the pane as the collapse handle.
1. Change the TinyMCE theme to [thebigreason](http://thebigreason.com/blog/2008/09/29/thebigreason-tinymce-skin). Should this one go into grappelli-safe instead?