Implement basic functional tests for client

We do not currently have any tests which actually exercise the core functionality of the client as a complete application. Despite good test coverage, it is still possible to break core functionality of the app without the issues being caught.

I envision a test setup which does something like this:

- Build the client and load it into a simple HTML page using a headless version of either Chrome or Firefox.
- Configure the client to talk to a dummy server (perhaps express-based) which returns canned responses for the test page's HTML
- Interacting with the application as a user would, attempt to:
- View a set of "existing" annotations
- Log in
- Switch to a group
- Highlight some text and click "Annotate"
- Fill in test content for the annotation
- Post the annotation