Show start and end voting date to voters in invitation email.

helios/templates/email/vote_body.txt

@@ -4,6 +4,10 @@ Dear {{voter.name}},
 
 Election URL:  {{election_vote_url}}
 Election Fingerprint:  {{voter.election.hash}}
+{% if election.voting_start_at %}Voting start at {{election.voting_start_at}}
+{% endif %}{% if election.voting_end_at %}Voting end at {{election.voting_end_at}}
+{% endif %}
+
 {% ifequal voter.voter_type "password" %}
 Your voter ID: {{voter.voter_login_id}}
 Your password: {{voter.voter_password}}