add a 'turn off sentry I'm doing development right now' env var

[jamesabel]

It's tempting to turn off Sentry during development and then forget to turn it back on. Need a simple way of turning it off during development. Clearing the DSN is a pain, since you then have to set it back to the correct (somewhat long) value string. Probably do something like use an env var:

set BALSA_DEV=TRUE

and use

balsa_dev = bool(distutils.util.strtobool(os.getenv('BALSA_DEV', 'FALSE')))

then:

use_sentry=not balsa_dev