Make enabling content_security_policy configurable via gdk.yml
What does this Merge Request do and why?
This MR adds the gitlab.rails.content_security_policy.enabled setting (enabled by default) along with accompanying gitlab/config/gitlab.yml template changes and assists with https://gitlab.com/gitlab-org/analytics-section/product-analytics/devkit#disable-cors.
Merge Request checklist
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This change is backward compatible. If not, please include steps to communicate to our users. 
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Tests added for new functionality. If not, please raise Issue to follow-up. 
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Documentation added/updated, if needed. 
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gdk doctortest added, if needed.
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Add the ~highlightlabel if this MR should be included in theCHANGELOG.md.
Edited  by Ash McKenzie