Show explicit warning when we add a new secret

What does this MR do?

This gives an explicit warning message for the concern https://gitlab.slack.com/archives/C02PF508L/p1592324786260900?thread_ts=1592319703.254800&cid=C02PF508L

the top of the file does mention adding it to Omnibus (although not charts!), but that's not easy to see for reviewers, and it's also quite easy for authors to miss: !34249 (diffs, comment 362234049)

Related #222690

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team