Add and refactor Administration menu

What does this MR do?

In this MR, we're refactoring the Administration menu. We're moving it from being a partial rendered menu to an object-driven menu, like all the menus in the project sidebar.

For that, we're just introducing a new object to store the menu info and removing the related info from the menu partial.

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

• Enable the feature flag group_administration_nav_item
• Most of the menus in the Administration menu requires some configuration to be set. For example:
  • ::Gitlab::Auth::GroupSaml::Config.enabled? needs to be enabled
  • group.feature_available?(:group_saml) needs to be enabled as well
  • ::License.feature_available?(:usage_quotas) is required
  • ::Gitlab::CurrentSettings.should_check_namespace_plan?

The easiest way to test this menu, although you won't be able to access the actual pages is to comment out:

• https://gitlab.com/gitlab-org/gitlab/blob/336968-fj-add-administration-menu/ee/lib/sidebars/groups/menus/administration_menu.rb#L44
• https://gitlab.com/gitlab-org/gitlab/blob/336968-fj-add-administration-menu/ee/lib/sidebars/groups/menus/administration_menu.rb#L57
• https://gitlab.com/gitlab-org/gitlab/blob/336968-fj-add-administration-menu/ee/lib/sidebars/groups/menus/administration_menu.rb#L74

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Related to #336968 (closed)