Add `report` flag for copyrighted material

[Zabriskije]

I've seen many icons with permissive/copyleft licenses shared or adapted without proper attribution, and copyrighted icons shared without explicit permissions by the copyright owner. So I propose adding a report flag with two functions:

• Request takedown — for copyrighted icons shared by users that have no legal right to do so
• Request attribution — for icons shared with permissive/copyleft licenses, missing proper attribution to the original author

Paternity is hard to enforce since there's no section to display copyright disclaimers. #1413 could be a solution to this, so attribution can be displayed on the icon's page itself. In addition, attribution should be made explicit in the front/search page, so the user can see that the shared icon comes from another author (or is a derivative work).

On the left, icon on hover with added report flag; on the middle, icon with a copyright info button; on the right, info button on hover (hover goes away when the mouse is outside the icon box):

  I know there's a mandatory I'm the author of this icon checkbox when submitting icons, but that doesn't take into account derivative works, and it seems not well reinforced.

If this is too much burden, I hope at least we can get an option to request takedowns.