Add "Home organization" setting to profile preferences

What does this MR do and why?

Related to #419079 (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/architecture/blueprints/organization/

Users can be members of multiple organizations but can choose one to be their "Home organization". This MR adds a setting to select your home organization in /-/profile/preferences.

Notes

• This dropdown is using mock data until GraphQL query for listing a user's organizations (#417892 - closed) is complete.
• Because the dropdown is using mock data the search and infinite scroll do not yet work. When we switch to a real API these will work.
• This setting does not persist to the database yet. We first need to complete Add user setting for "Home organization" (#428668 - closed)

Screenshots or screen recordings

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 /-/profile/preferences

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.