Frontend (GitLab Secrets Manager): Create root Vue component and use `vue-router`

Summary

This is the second part of setting up the base code for the frontend work for the Tanu-Key MVC.

This issue will set up the basic structure of our frontend code. This includes the following tasks:

• Create the root Vue component and mount this on our HAML template.
• Use vue-router to manage routes. This should show up for the / and wildcard routes.
• Set up Apollo. For now, we will use client resolvers and mock the data we need for development.
• Set up our base directory structure for frontend.

Links/References

Please refer to the main frontend epic for more information.