Add validation to image_name in notebook server creation to prevent or strip leading "http[s]://"

[ca-scribner]

When creating a notebook server you can enter an image name that has a leading http[s]://, but this will never work (the name used here is a docker image, eg: [repo/]imagename:tag). The frontend can validate this input and do one of:

• strip leading http[s]:// from any image name without the user knowing it had to be done (preferable unless there's a use case that ever needs the leading http)
• warn the user when filling out he form (or reject outright) that http is not accepted (do this if we don't think we can just safely always strip)