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
Apply the same spam checks to public snippets (either personal snippets
that are public, or public snippets on public projects) as to issues on
public projects.
Add new service classes to create and update project and personal
snippets. These classes are responsible for enforcing restricted
visibility settings for non-admin users.
