Pass environment to Sentry

[jazairi]

Why these changes are being introduced:

All Sentry alerts currently display the environment as 'production'.
We would like Sentry to accurately show the environment in which
the alert was captured.

Relevant ticket(s):

https://mitlibraries.atlassian.net/browse/ENGX-181

How this addresses that need:

This sets the Sentry environment to SENTRY_ENV, defaulting to
'unknown' if that variable isn't set.

Side effects of this change:

None.

Developer

• All new ENV is documented in README
• All new ENV has been added to Heroku Pipeline, Staging and Prod
• ANDI or Wave has been run in accordance to
  our guide and
  all issues introduced by these changes have been resolved or opened as new
  issues (link to those issues in the Pull Request details above)
• Stakeholder approval has been confirmed (or is not needed)

Code Reviewer

• The commit message is clear and follows our guidelines
  (not just this pull request message)
• There are appropriate tests covering any new functionality
• The documentation has been updated or is unnecessary
• The changes have been verified
• New dependencies are appropriate or there were no changes

Requires database migrations?

NO

Includes new or updated dependencies?

NO

[jazairi]

All Heroku apps have been updated with the corresponding environment.