Checks for always flag on the following nginx headers:

• Strict-Transport-Security
• X-Content-Type-Options
• X-Frame-Options
• Content-Security-Policy
• Content-Security-Policy-Report-Only

Closes #1527

What has been done to verify that this works as intended?

• added a semgrep rule

Why is this the best possible solution? Were any other approaches considered?

• a similar check could be achieved with grep. semgrep may be more versatile, and pulls in community-provided checks
• unit tests could be added instead/as well

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

No effect.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No.

Before submitting this PR, please make sure you have:

• branched off and targeted the next branch OR only changed documentation/infrastructure (master is stable and used in production)
• verified that any code or assets from external sources are properly credited in comments or that everything is internally sourced