Remove personal_homepage feature flag

What does this MR do and why?

The personal_homepage feature flag has been fully rolled out and is now the default behavior for all users. This change removes the feature flag definition and all related checks throughout the codebase.

The personal homepage with flipped dashboard mapping is now always enabled for all users (except self-managed admins with no projects, who follow a different onboarding flow).

Changes include:

• Removed feature flag definition file
• Updated User model to always use flipped dashboard mapping
• Updated controllers (Root, Dashboard, Users) to always render homepage
• Updated helpers and sidebars to always include homepage functionality
• Updated JavaScript to always initialize homepage
• Removed all feature flag stubs from specs
• Updated QA tests to remove feature flag usage

Changelog: changed

Closes: https://gitlab.com/gitlab-org/gitlab/-/issues/577448

References

Screenshots or screen recordings

Working as intended

SM Flow:

SaaS flow:

How to set up and validate locally

Example: SM flow:

1. export GITLAB_SIMULATE_SAAS=0 gdk start
2. login as root, you will see onboarding screen (given that you have no authorized projects) if you have authorized projects, you will see homepage.
3. To see the onboarding screeen, from the console, destroy all projects Projects.all.destroy!

SaaS

1. export GITLAB_SIMULATE_SAAS=1 gdk start
2. login as root, you will see the homepage

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.