Make /solved clear the state string for a puzzle

Make /solved clear the state string for a puzzle

In the last test we run, we often saw stale state messages about a
puzzle being half-solve, etc. when it was actually already completely
solved. Instead of putting another chore onto solvers to have them
manually clear the state string after solving a puzzle, here we do
that automatically.

This completes another low-hanging fruit item from the TODO list.