Add mark_onboarding_complete mutation and FE files for pages pipeline wizard

This commit introduces a GraphQL Mutation allowing
a once-off toggle of the "onboarding_complete"
property in Pages metadata.

This commit introduces the frontend for an onboarding view for Pages
that will enable the user to configure their .gitlab-ci.yml file
directly in the UI.
This commit needs a follow up MR to be displayed by the UI.