Add dashboard route for new homepage

What does this MR do and why?

Adds /dashboard/home route for new homepage.

While we want this new homepage to live at / by default, we will still have the user preference that allows users to set their prefered homepage to other targets (like the todos dashboard, groups index etc).

If a user customizes their homepage target to point at something else, the root route does a redirect_to that target. In this scenario, we cannot just use / as link to the new homepage alone, but it needs its own unique route. /dashboard/home fits nicely into our existing naming pattern.

References

This is a small bugfix as part of Add new navigation item for the personalized ho... (#545450 - closed).

How to set up and validate locally

• Enable the personal_homepage feature flag and visit /.
• Assuming your user uses the default option for the Homepage user preference, you will see the Projects index as homepage at /.
• Use the left sidebar to navigate to Home. This link now points to /dashboard/home instead of /.
• You should see the new homepage.

MR acceptance checklist

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