GitLab Guest User should not be able to view GOUI

Based on the discussion in &36 (comment 1209563153)

GitLab Guest users should not automatically be a GrafanaViewer. While this may be an allowed situation, until we have a solution to prevent unwanted views of GOUI, let's remove a guest user from viewing GOUI.

The same permissions model will apply to both viewing the GOUI and viewing embedded charts.