Allow to set sentry environment via env var SENTRY_ENVIRONMENT

[wozniakpl]

Checklist

1. If you've added code that should be tested, add tests
2. If you've changed APIs, update (or create!) the documentation
3. Ensure the tests pass
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5. Write a title and, if necessary, a description of your work suitable for publishing in our release notes
6. Mention any related issues in this repository (as #ISSUE) and in other repositories (as kobotoolbox/other#ISSUE)
7. Open an issue in the docs if there are UI/UX changes

Description

Updated Sentry initialization to dynamically set the environment parameter using an environment variable SENTRY_ENVIRONMENT. This aids in filtering and analyzing errors by deployment stage.

Notes

Added this line to sentry_sdk.init call's params:

environment=env.str('SENTRY_ENVIRONMENT', 'production'),

It's set by default to production for backwards compatibility purposes. This is what's set by default.