Redirect root to new Vue to-do app if feature flag is enabled

What does this MR do and why?

Now, also navigating to / (by clicking the top-left tanuki logo) will redirect to the new Vue to-do app (given you have todos set as your homepage, of course).

We do the same already for the links in the sidebar, but forgot to also change the redirect_to behavior in the root controller the same way.

We will revert all these if-statements (here and in the sidebar code) when we release the new app for everyone under the old URL. This extra ./vue route is temporary while this is in internal testing.

MR acceptance checklist

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

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.