Better placement for label editor on issues page

[miketaylr]

As suggested by @karlcow in #630 (comment).

[miketaylr]

@magsout has some good ideas that might make this issue obsolete in #545.

[miketaylr]

Action item: implement side label widget thinger, consider ways this makes sense on mobile.

One proposal is to show labels on top and bottom (for mobile viewport).

[karlcow]

After discussing with @adamopenweb and @magsout, we decided to use icons at the top for the browsers. And to have actions near the comment box.
The large screen would get label editor on the right side. Small screens at the bottom.

[miketaylr]

This is done.