Ensure personal snippets are isolated within Organization

Personal snippets are instance wide but need to be made Organization wide by being brought into the Organization visibility rules.

The Organization visibility should:

• Limit the visibility options of personal snippets.
• Migrate visibility of personal snippets.
• Ensure that personal snippets are not accessible when a non-Organization member tries to view the URL.

Implementation plan

Personal snippets are already sharded by Organization. The only work necessary for this issue is to limit/validate visibility level based on Organization visibility level. There may be 2 separate pieces if the work isn't combined into 1 MR - one frontend and one backend piece.

• Weight: 3
• Issue / MR breakdown:
  • frontend MR to update app/views/shared/snippets/_form.html.haml to only present the available visibility levels. The form is a Vue component but the available visibility levels is already extracted into a helper method.
  • backend MR to add visibility validation on the model