Adding sentry to the installation

[cataclysmic]

I learned that sentry is a django based open source project and can be installed locally.
I will look into it and we will add it to the installation instructions and add the necessary stuff to the otree_environ_config.

Todo

• pip install -U sentry on initialise (optional)
• write sentry link to otree_environ_config for each user
• on otree_startproject ask if local sentry should be activated (static)
  • run sed on settings.py, add SENTRY_DSN = environ.get('SENTRY_DSN')