adapt event issue template

[Brilator]

Once the schema is fixed, the template needs to be updated and properly described.

[Freymaurer]

• This does not matter?
  

• The template does not list the option for an external link

• The template might require a "note for the editor" field. In which users could directly address the event editor if they are unsure about fields or where to place what information

[Brilator]

The template might require a "note for the editor" field. In which users could directly address the event editor if they are unsure about fields or where to place what information

I'd suggest to just use GitHub logic for that.

[Brilator]

This does not matter?

Yes, I'll fix the validations for mandatory fields.