Fix a few minor UI bugs

First off I'm pretty welcome to feedback of all sorts.
The "hack" of adding 3 px I think was from before `tolerance: 'pointer'` was used or available from jQuery UI. Removing it didn't do much, but it made the reorder indicator jump to the beginning of the table instead of the end when you were hovering to switch with the last column. So I looked into it, and there was a `ui-state-highlight` that was taking up a little bit of space. It wasn't actually being displayed, only used as a point of reference for the `.ember-table-column-sortable-indicator`. So I made it zero-width.
What I'm a bit less sure about is changing `this.$().parent().outerWidth());` to `this.$().innerWidth()`. I was afraid it would create some sort of bootstrapping problem, but everything looked fine in my testing.