Enable proper subject lines in contact page emails

[ardalis]

Currently emails sent through the contact form all have a subject:

Instead the submitter should be able to provide a subject, and the subject should be used instead of "Form submission...". It's fine if it still starts with "[Netlify]".

There may be some solution in this thread:
https://answers.netlify.com/t/custom-subject-for-form-email-notifications/560

Also the docs suggest it's quite simple:
https://docs.netlify.com/forms/notifications/