Co-authored-by: nd <nd@cccv.de>

Broken mail clients like Thunderbird fail to recognize urlsafe characters
like "-" as part of an URL. token_urlfriendly avoids those characters.

Closes #93.

Co-authored-by: nd <nd@cccv.de>

The output of the "qrcode_svg" template filter emmitted a tag with a constant
id. When used multiple times in a single document (i.e. the invites list),
this leads to multiple elements sharing the same id value.