Simplified the voting process (for Bitcoin Foundation).

Hi Ben! We are using Helios at the Bitcoin Foundation for elections. We had some issues with the UI, and I fixed them in this fork by removing steps in the voting process while retaining full functionality. Can we get these changes merged into your main repo?

Here's the commit message:
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These changes solve the problem where users would erroneously think their
ballot was cast before completing all steps in the process, and makes
the user experience simpler and smoother for all users.

Specifically, this change eliminates one or two steps (depending on
whether the user is logged in), while retaining full functionality:

  - Merges steps 2 and 3 on the client (show_confirm and seal_ballot)
    into a single step 2.

  - Improves the formatting of the confirmed ballot in step 2 to make
    it easier to understand.

  - In the user interface, "step 3" now means the SERVER-SIDE step of
    submitting your ballot to the helios server.  Previously, this
    step was not expressed within the [1,2,3] progress bar in
    helios-booth, which contributed to users thinking they had
    finished voting even though they had not finished this "hidden
    step 4" of submitting to the server.

  - If the user is already logged in when they reach this server-side
    step 3, we now submit the ballot automatically, rather than
    requiring the user to yet-again click "submit the ballot."  We
    only require further action for users who are not logged in.