Enable mailto links

We had a discussion about potentially enabling mailto links.

I don't remember what was the conclusion.^^"

@pierov do you remember?

Design estimate:

• Complexity: small (1 days)
  • List out potential privacy issues of enabling MIME handlers.
  • Design a dialog (using existing Firefox popups) that tells the user about it.
  • Consider adding an option as well for toggling this option.
• Uncertainty level: moderate (1.5)
  • There is bound to be a discussion about the copy.
• Total: 1-1.5 days