Fixed scrollbar positions

I moved the main content scrollbar below the main toolbar and introduced a new component for layouts with two scrollable columns. The scrollbar for the left column is now directly next to it. Unfortunately i've had to hardcode the padding on the main wrapper to be equal to the main toolbar height, so it might break if the toolbar height ever changes in a vuetify update, but i think that's a compromise we can make. (Vuetify 3.0 will expose the toolbar height as a variable: https://github.com/vuetifyjs/vuetify/issues/9532)

Closes #260 (closed)