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