fix: prevent eslint errors from blocking build

Currently, when we attempt to start our react app, create-react-app
fails to compile when there are any eslint or prettier errors. The
reason for this is documented in this github issue: facebook/create-react-app#9887 (comment)

This issue is fixed with react-scripts v4.0.2 with the use of an
ESLINT_NO_DEV_ERRORS env var (this is documented under advanced
configuration here: https://create-react-app.dev/docs/advanced-configuration/)

This commit adds the ESLINT_NO_DEV_ERRORS env var to our env-example
file. We will need to separately add it to our production and staging
environments too.

.env-example

@@ -3,6 +3,7 @@ export REACT_APP_BACKEND_URL=''
 export REACT_APP_SENTRY_ENV=''
 export REACT_APP_SENTRY_DSN=''
 export REACT_APP_ENV='LOCAL_DEV'
+export ESLINT_NO_DEV_ERRORS=true
 
 # Cypress test environment variables
 export CYPRESS_BASEURL=''