Notify Bugsnag on Invalid Submission - close #1719

Closes #1719

Add a custom "data" tab to the bugsnag message with the current
username, code, and path from the request for additional debugging
resources.

This can be tested locally with the following steps:

1. Register for bugsnag

2. Add BUGSNAG_API_KEY=your_personal_key to the `.env` file at the
project root

3. Run the project server with `$ foreman start`

4. Make a curl request like to this (key can be any user's key):

```sh
$ curl
  -X POST
  -w "\n"
  --data '{ "key": "vdv961",
            "code": "some code \n more code ... \n and more code",
            "path": "/some/path"}'
  "localhost:4567/api/v1/user/assignments"
```

... and the error should now be viewable on Bugsnag.