Install Sentry (backend)

[skyqrose]

Asana Task: Other basic backend infrastructure required to build a basic HR Glides app

Could be reviewed but doesn't have to be yet. Ready for code review and merging!

Infra PRs for setting the ENVIRONMENT env_var:

• https://github.com/mbta/terraform_modules/pull/18
• https://github.com/mbta/devops/pull/1901

Edit: I pulled most of the work of setting the release into #26

Still to do:

• Make terraform PRs to set new env vars for ENVIRONMENT and SENTRY_DSN.
• Write tests. I think nothing left here is unit testable.
• Deploy to sandbox and check that it works.
  • In particular, check that source maps work, I'm still not sure about them.
• Remove the DO NOT MERGE commit that allows me to trigger crashes.
• (Probably not in this PR) once we have keycloak users, register the user as sentry metadata on each request.
  • edit: Asana Task

This PR includes quite a lot:

• New dev.secret.exs and dev.secret.example.exs files (which I used for setting sentry config locally).
• Installs sentry, including releases and source maps.
• Adds a release variable to the app.
  • In Glides, this is SENTRY_RELEASE or Application.get_env(:sentry, :release), but then we started using it for other stuff, so I've just gone straight for a generic RELEASE or Application.get_env(:orbit, :release).
• Adds the release to the HTML head.
  • So that it's possible to verify that the release is getting set correctly.
  • This is definitely testable, I just haven't yet.

Checklist

• Tests:
  • ( ) Has tests
  • (x) Doesn't need tests. Nothing left here is unit testable.
  • ( ) Tests deferred (with justification)
• Product/Design sign off:
  • ( ) Okayed the plan for the feature (e.g. the design files, or the Asana task)
  • ( ) Reviewed the feature as implemented (e.g. on dev-green, or saw screenshots)
  • (x) No review needed

[skyqrose]

Sentry reporting worked, and there's source maps: https://mbtace.sentry.io/issues/5487566542, https://mbtace.sentry.io/issues/5513834067

[skyqrose]

Okay, this is ready for review now!