T229 appbar redesign

Updates the Appbar to match this prototype. Additionally, it:

• Updates react-router and react-router-dom to 5.1.2 (So we can use the useHistory, useLocation etc hooks)
• Removes the dependency on use-react-router (because now official hooks are available)
• Removes the ProjectToolbar component

Notes on new Appbar structure:

The src/components/Appbar/Appbar.js component now has routes inside it to determine which page specific appbar should be rendered. I have added src/components/Appbar/Appbar.project.js as our first page-specific appbar - as we make more we should discuss how we might reorganize this component folder to better handle that, but with just one or two I think it's fine to keep them in the Appbar folder.

I additionally added a helper component in src/components/Appbar/AppbarButtonContainer.js, which is to be used for displaying buttons in the appbar. It automatically converts buttons to an overflow menu when there's not available space. See src/components/Appbar/Appbar.project.js for an explanation of how it's used.