Improve list name validation for the new list form
When creating a new mailing list, if the list name is not correct (e.g. contains space character) the error message states "Please enter a valid listname".
For the new user it might be not clear why the entered string is not valid and that the list name will be actually used as a part of email address.
I suggest adding a help text explaining this explicitly and also specifying accepted characters range.
Edit: Plan is to have Core provide a validation function and us that, so this is currently blocked by mailman#339