Add association counts to organization front page

What does this MR do and why?

Related to #421447 (closed)
Designs: &11187 (closed)

The grouptenant scale team is starting to build out an Organization MVC. For more information about what an "Organization" will be see https://docs.gitlab.com/ee/user/organization/

This MR continues starts building the front page by adding the associations count section. More progress on the front page to come in &11187 (closed)

Engineering notes

• These counts are currently using hardcoded mock counts. We need backend support before we can add real counts. That will be tracked in #424531 (closed)
• The Manage link in the users card doesn't work yet because we haven't setup the Users page yet. That will be completed in #409313 (closed)

UX notes

• There is a bug in the sidebar due to a recent global sidebar change. Asked in #396719 (comment 1548062233) about how we should fix and I will open up a MR later.
• There will be some minor adjustments to the group and project list in #420800 (closed)
• The Organizations::Organization model does not yet support a description so we will be adding that in #422078 (closed)
• Subscriptions will not be part of the MVC so I have left it out of the avatar. Confirmed with PM &11187 (comment 1514168095)

Screenshots or screen recordings

Desktop	Mobile

How to set up and validate locally

1. Open Rails console
   • bin/rails console
2. Enable the feature flag
   • Feature.enable(:ui_for_organizations)
3. Navigate to /-/organizations/default

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.