Suggest slug on /new channel

[oskarrough]

When you create a new channel we should make sure the slug is "slugified" e.g. lowercase and all that jazz.

• see <r4-channel-create>
• https://github.com/internet4000/radio4000/blob/69f6de23cf17ef6931919a27a4235ea6805bfece/app/utils/slugify.js#L7