Move SENTRY_DSN to a build environment variable.

[DropsOfSerenity]

Moved the sentry dsn to a built environment variable. This keeps it out of the repository, which:

• Ensures that developers starting with our template don't end up using our dsn.
• Makes it optional easily.
• Ensures that we don't have sensitive credentials in our repo.

We also no longer initialize sentry if we don't have a SENTRY_DSN