Add embeddable charts feature flag

What does this MR do?

This MR introduces the analytics_embeddable_charts feature flag to be used for the embeddable charts feature: #213924 (comment 361159611)

It covers the projects/insights_controller with tests as well as add tests for the added feature flag.

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

I don't really see much risk in this MR as it just uses already proven-to-work infrastructure, and it tests the new behavior.

• 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

Related to #224174 (closed)

Related to !35535 (merged)