Send E-Mail When Voting Period Starts

New option for the creation of an election: Select whether an e-mail should be sent to all participants when the voting period starts. This feature is important for the Hausadmin election.

• Add option in the election model whether an email should be send
• add a check for this option when an election is opened and send emails
• we might also need to add a separate check when an election has a start time and therefore is not actively triggered
• add a template feature for the election creation to customize the email text
• the template feature from above should also be possible for a whole session in order to customize the invitation email text
• add systemd timer in the deployment