Set up basics of React Router

We will be using React Router for this project. We need the basics of the router set up to handle the different routes, using the basic app structure:

```
<App>
  <Navbar/>
  <router>
    <LandingPage/> (when not logged in)
    <LogIn/>
    <SignUp/>
    <UserDashboard /> (logged in)
    <TripBuilder/> (logged in)
  </router>
</App>
```