Verifiability - add users' hash of the encrypted ballot for each election.

[chenchanglew]

Implementation step of Verifiability Documentation

1. frontend: show a user the hash of their encrypted ballot. (Issue: Verifiability - show vote hash after casting a vote #239)
2. smart-contract + proxy: add users' hash of the encrypted ballot (sha256) in the form. The hash will send along with the getForm to the user. (This issue)
3. frontend: In the election info page, show a list of the voter who vote and their hash of encrypted votes got from the proxy. (Issue: Verifiability - show hash of encrypted vote in election detail pages #241)

How:
Update the form struct by adding an array variable to store the hash of encrypted votes.
Update the getForm function to return the hash of encrypted votes.

Where:
contracts/evoting/types/election.go
proxy/election.go getForm()

Target:
edit the unit tests in proxy, and update the getForm unit-test.
edit the unit tests in smart-contract, update the form related unit-test