Add Integrations Menu Item and List to Group Settings

• Within Group > Settings, user should be able to see a new section in the navbar for Integrations.
• The integrations page should show all available integrations that can be setup for the group.
• Each item in this list should link to a page to create the integration, which will be handled in #209825 (closed)
• All of this should be behind a feature flag group_level_integrations with options to allow it per-project and / or per-user.
• We should reuse as much as code as possible and possibly refactor / extract common functionality where needed. For example, app/views/projects/services/_integrations.html.haml (found here) can be used for the listing of integrations.
• There should be a link to the documentation, describing how group-level integrations work.