Refactor Security Dashboard store

What does this MR do?

Refactors Security Dashboard store.

This refactoring is part of the Instance Security Dashboard MVC, to abstract away from the base Security Dashboard the logic for how the list of projects to filter with is set. It:

• Adds a new GroupSecurityDashboard component that wraps the SecurityDashboard component.
• Moves the projects module bindings/calls from the SecurityDashboard component to the new GroupSecurityDashboard.
• Creates a new projects plugin for the Security Dashboard store which dynamically adds the projects module and sets up the appropriate store bindings between it and the filters module.
• Updates the entry point for the Group Security Dashboard to add the projects plugin to the Security Dashboard store.
• Moves the existing mediator and sync_with_store plugins into a plugins directory.
• Renames moderator to mediator.

Screenshots

N/A; this should not result in any visible changes to the Group, Project or Pipeline Security Dashboards.

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