[UX] Addon install warnings based on behaviorHints

[Ivshti]

When installing an addon for the first time, show warnings in the install prompt if there are any of the manifest.behaviorHints

• adult: show a warning "This addon contains adult content. Please check the laws in your jurisdiction. By pressing Install, you confirm you are of legal age."

• p2p: show a warning "This addon may contain streams from P2P protocols such as BitTorrent. Please check the laws in your jurisdiction."

• [ ] design the warnings/disclaimers - those should be a part of the install dialog and appear between the description and the buttons

• [ ] refine those messages, check with a lawyer

• [ ] implement