Small fixes on `ww-code`

[salmenf]

Some might be fixed already in Jorik's local version, just listing all I noticed in ww-code@0.2.0:

• *Allow code execution (typo)
• Switching the exercise type does not work ("No exercise type" is always selected)
• Visual glitch on border of editor outline
• Show lock button preview when hovering over the line gutter
• HTML preview iframe sizing (this is just from our meeting, it's not in 0.2.0)
• Optimize vertical spacing even more:
  • Move language selection into the bar with "run" and "save", still aligned right
  • Remove horizontal rule/line between selection of exercise type and the toggles
• Remove "Result: " -> Just show the result of there is one
• The run and save buttons should be styled differently on :hover and on :active, same for the language selection
• If code execution is not possible for the language, the run button should be greyed out and disabled