possibility to manually generate voter access/invite codes

The idea is to be able to manually generate voter invite links / access codes, to e.g. distribute them via letters / QR codes

• add option to the election UI to generate a list of access codes
• make the email field on the Voter model optional since this method does not require us to send links via email

Optionally

• add option to download generated links as PDF with QR codes
• ask manager before the qr code pdf is generated to inform him that the codes will be rendered invalid