Final tweaks to admin navigation panel

These are the final tweaks to the admin navigation pane. I changed the style of the border but left the 7px width, so now the border looks consistent with the rest of the panel and really makes you want to click it :P

In case you want to change the width or the look of it (and for future reference), bear this in mind:
- The navigation menu + the border must add up to the width of the top header. Remember width is computed without the border (because we're not using `box-sizing: border-box`), so the total width of the panel is `width of menu` + `1px menu right border` + `1px handle left border` + `width of handle` + `1px handle right border`.
- When you toggle the pane, three things happen: `.dropdown-menu` is hidden (by adding the `hidden` class), `#content` is stretched (by adding the `full` class), `.messagelist` is stretched (also adding the `full` class). Yes, it has to be this complicated because we use fixed positioning in several places.

Sorry for smashing this on top of my previous commits, I messed up the merge.