Separate Dev and Production for Sentry

### Difficulty
Medium (1-2 people)

### Description
Currently all dev errors are showing up on our Sentry (bug reporting) page. Create a dev environment that would disable Sentry. To do this, first create a "dev" script in the `package.json` that works just like the "start" script but add an environment variable such as "DEVELOPMENT_MODE" or something.

Use this environment variable to disable the Sentry wrapper in `SentryWrapper.tsx`. So now if you start the project with `npm run dev`, Sentry will not be online.

Finally, update the README to suggest the new dev script instead of the start script.