Get Trustees opt-in before doing KeyGen

[dsernst]

The idea here is to rejigger the Verifying Observers interface, so that when they load the page it doesn't immediately begin Keygen, but instead explains to them what their role is, and asks them to confirm they'll have access to their device at the end of the election.

[dsernst]

Found these old notes on this:

• [ ] Ask trustees to confirm before they keygen

  • [ ] Prompt for device name?
  • [x] Show device user agent?

• [ ] Warn before letting gMail app keygen

  • [ ] https://stackoverflow.com/questions/9077101/iphone-localstorage-quota-exceeded-err-issue