Event cannot be accesed via subdomain when using an event tag that isn't compatible with subdomains

[Victor4X]

Describe the bug
This is more of a UX bug, but can be annoying/confusing.

When creating an event, the event tag can include uppercase letters. These get normalized to lowercase when used in the <event-tag>.<hauukins-site> subdomain, leading to an "event not found" error page. I have confirmed that sending the correct event tag (with uppercase letters) to the api.<hauukins-site>/v1/event/<event-tag> endpoint correctly resolves the event.

I have not tested further, but there might be other strings that are accepted by the form, but don't work in practice.

To Reproduce
Steps to reproduce the behavior:

1. Go to /events/new in the admin panel
2. Insert a string with uppercase letters in the "Event tag" field
3. Create the event
4. Click "go to event" button
5. See error
6. There is no way (that I could see) to access the event

Expected behavior
An error when submitting the form that event tags have to be subdomain-compliant