A good looking E-Mail template

**Is your feature request related to a problem? Please describe.**
I would like to have a decent looking E-Mail template, so that the E-Mails with the verification files don't look like spam.

**Describe the solution you'd like**
A simple HTML / CSS template which can be used by `webhook.go`

**Describe alternatives you've considered**
Let it be

**Additional context**
I would have to read up if we can pass it via file or otherwise.
It should look pleasing to the eye!

The issue names webhook.go as the entry point and requests an HTML/CSS template for verification-file emails. Read webhook.go first to see how those emails are assembled and whether a file-based body is supported. Done means the verification emails use a readable, non-spam-like template.

Written by the indexing model from the issue text.