[enh] Confirm when installing experimental or low quality apps

[alexAubin]

Problem

Currently there is no warning shown when installing experimental or low quality apps. We should add some to discourage users installing apps likely to break their system ...

Solution

• Ask for confirmation when installing low quality or broken/experimental apps
  • A "good quality" app is defined as working + level >= 3
  • A "low-quality app is defined as working + level = 1 or 2
  • A broken / experimental app is all other apps
• Increase spookiness of message when installing apps from custom URL