Add Instance-Level Integration Settings UI

What does this MR do?

Issue #196525 (closed)

Behind a feature flag instance_level_integrations.

• Move current section with "integrations" to Admin > Settings > General.
• Add note of the move of the old settings.
• Add new Integrations table inside Admin > Settings > Integrations. Add a partial that will be re-used in other views for the integrations table.
• Add dummy data (with project integrations) to show the table.

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