Fix snippets API not working with visibility level

When a restricted visibility level of private is set in the instance, creating a snippet with the visibility level would always fail. This happened because:

1. params[:visibility] was a string (e.g. "public")
2. CreateSnippetService and UpdateSnippetService only looked at params[:visibility_level], which was nil.

To fix this, we:

1. Make CreateSnippetService look at the newly-built snippet.visibility_level, since the right value is assigned by the VisibilityLevel#visibility= method.
2. Modify UpdateSnippetService to handle both visibility_level and visibility parameters.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/66050