Fix several warnings like this when running tests:

```
22 06 2020 14:01:32.661:WARN [middleware:karma]: Unable to determine
   file type from the file extension, defaulting to js.
     To silence the warning specify a valid type for
     /Users/robert/hypothesis/repos/client/src/annotator/test/sidebar-test.coffee
     in the configuration file.
       See http://karma-runner.github.io/latest/config/files.html
```