Split border-width into separate variable

In order to override some content spacing based upon header/footer bars, it was necessary to take border-width into account during other sass calculations.  Splitting it into a variable improves readability instead of having code like:

```
.content {
    margin-bottom: ($bar-base-height + ($bar-side-spacing + 1) * 2);  // adding 1 for $border-default's border-width
}
```