This refactoring is part of the [Instance Security Dashboard MVC][1], 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`.

[1]: https://gitlab.com/gitlab-org/gitlab/issues/6953