Add feature flag and user preference for new Personal Homepage

What does this MR do and why?

Adds a new personal_homepage feature flag and user preference choice for "Personal homepage" as their prefered / page.

References

• Epic: Personalized Home Page > Minimal maturity (&16657)
• Issue: Homepage > Add user preference option (behind n... (#546151 - closed)

Screenshots or screen recordings

The new page is still blank, and it does not yet show up in the left sidebar nav:

How to set up and validate locally

Most important: Test the feature flag disabled, as this is what our users will see for now.

• Test going to / with the default homepage preference (http://gdk.test:3000/-/profile/preferences Behavior/Homepage)
• As before, you should see the Projects dashboard (first tab), and the path should still be /
• Test other homepage options. These should work as before, doing redirects.

Test with FF enabled:

• First, test that the default behavior is still the Projects dashboard. We want to keep the new homepage opt-in while it is in its early dev phase.
• Now, check http://gdk.test:3000/-/profile/preferences and change your prefered hompepage to the new option Personal homepage (WIP).
• Now, visiting the root URL / should show you a blank page (but with correct "Home" breadcrumb).
• The sidebar nav does not yet reflect that and still shows "Projects". I will fix that in a later MR to keep this one simple.

MR acceptance checklist

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