ui: Fix "open in browser" dialog for single window mode
ui: Add options to hide the browser nagging dialog

The "open in browser" option on links opens a dialog, meant to warn
users away from using their default browser to open links where they
might want to preserve anonymity.

This commit changes that UI, and adds options to skip that dialog in the
future for links from the specific contact, or for links from all
contacts.
Merge pull request #62 from special/open-browser

Fix and improve "Open Browser" dialog