Fix sending emails - Githubissues

Proposed changes

Fixed event registration emails not being sent
Improved wording and added image links to the emails sent to users after registering for an event
Emails are printed to console instead of being sent, when testing or debugging

None in particular.

(If any of the points are not relevant, mark them as checked)

[x] Created tests that fail without the changes, if relevant/possible
[x] Made sure that your code conforms to the code style guides and that any common code smells have been addressed
- (It's not intended that you read through this whole document, but that you get yourself an overview over its contents, and that you keep it in mind while taking a second look at your code before opening a pull request)
[x] Added sufficient documentation - e.g. as docstrings or in the README, if suitable
[x] Added your changes to the "Unreleased" section of the changelog - mainly the changes that are of particular interest to users and/or developers, if any
[x] Added a "Deployment notes" section above, if anything out of the ordinary should be done when deploying these changes to the server