Don't require users to type in a slug on WorkspaceCreateForm

[mikerkelly]

The WorkspaceCreateForm (/{project_name}/new-workspace/) has a field, name, which must be a valid slug (a short label suitable for use in a URL, containing only letters, numbers, underscores or hyphens). If the user does not enter a valid slug, the validation fails server-side and the form reloads, which is slow (see #4685). It's not so-reasonable to require a human to type out a slug, or to require two trips to the form if they make a mistake. The UX here could be improved. #4682 added some help_text explaining the requirement, but this seems unsatisfactory.

Suggestion: Apply django.utils.text.slugify in clean_name() or wherever, to convert what the user entered to a valid slug. Presumably the issue is usually they have used spaces in the name, which is easily fixed while maintaining their intent. We already lower-case the name.

Alternatively/additionally the client-side JavaScript could prevent entering non-slug characters by adding an onkeypress event listener.

Or maybe name should be free human-readable text and the slug should be separate.