Added todos_vue_application feature flag

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What does this MR do and why?

This MR introduces the todos_vue_application feature flag needed to switch between the legacy todos page and the new todos page written in Vue 🙂

Instead of interweaving the old todos app on /dashboard/todos with the new Vue app, we create a new route /dashboard/todos/vue which is a little more green field.

This will allow us to work without the CSS and JS tech-debt of the old page. Once the implementation of /dashboard/todos/vue is a little more progressed we can feature flag it on the route level. But in the mean time it's also nice that people which have the FF enabled, can just navigate to the old route as well.

Follow-up steps would be switching out the route based on the FF in the nav as well.

Changelog: added

Related to #464069 (closed)

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Screenshots or screen recordings

With Feature Flag enabled:

How to set up and validate locally

1. Without enabling the feature flag, /dashboard/todos/vue will redirect you to /dashboard/todos
2. When enabling the todos_vue_application FF (e.g. via /rails/features), /dashboard/todos/vue will render just a header and an empty div.