This commit contains the following changes:
* Better error handling: Previously, there was no proper handling in
of email-related errors. Now, all error messages are caught and the
user gets a nice landing page. The script will always try to contact
the organizers of the wedding with information on the error.
* Emails for not-attending: This commit adds the possibility to send
seperate emails to guests that attend and guests that cannot attend.
* Language strings for JavaScript errors: Previously, all JS errors were
in Dutch. Now, they are configurable.
This commit includes a fully functional template and Rust backend. The
website is able to send e-mails, displays strings in multiple languages,
include attachments, and has basic protection again spam (IP based).
