Add link to Instance Security Dashboard in nav

What does this MR do?

This adds a nav link for the Instance Security Dashboard MVC, which is behind the security_dashboard feature flag. The Instance Security Dashboard is also behind an as-yet undefined read_security_dashboard policy. For the screenshots in this MR where the Security link is visible, both the security_dashboard flag was enabled, and the following patch applied to enable the policy:

allow_read_security_dashboard.patch

A future MR will define the read_security_dashboard policy.

This MR also refactors slightly how the visibility of the Operations and Environments dashboard links is handled. One result of this is that they are now consistently visible/hidden between the dashboards dropdown on larger screens, and the "More" dropdown on smaller screens.

Finally, the partials have been renamed, since they're no longer specific to operations.

Screenshots

State	Before	After
All enabled, Dashboards dropdown
All enabled, "More" dropdown
Operations enabled, Environments enabled, Dashboards dropdown
Operations enabled, Environments enabled, "More" dropdown
Operations enabled, Environments disabled, Dashboards dropdown
Operations enabled, Environments disabled, "More" dropdown
All disabled, Dashboards dropdown
All disabled, "More" dropdown	Note: Operations link is displayed, which is incorrect, and fixed in this MR
Only Security enabled, Dashboards dropdown
Only Security enabled, "More" dropdown	Note: Operations link is displayed, which is incorrect, and fixed in this MR

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Performance 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