Add option for generated PANDA secrets.
Add option for generated PANDA secrets.

This changes the working in the new contact UI and adds a "Generate"
button in the PANDA flow. This is to support exchanging shared secrets
via email or IM which, while not as secure as other options, is a lot
easier.

ioerror/pond,agl/pond,ArneBab/pond,sillvan/pond,ArneBab/pond,infinity0/pond,agl/pond,sillvan/pond,berkus/pond,saltthefries/pond,infinity0/pond,saltthefries/pond,ioerror/pond,berkus/pond