Fix groups and stages detection

What does this MR do and why?

I noticed groupgitaly MRs would lead to Sentry errors (https://sentry.gitlab.net/gitlab/triage-ops/issues/3442045/events/761a24ed092a4682b42c2780fe2b3e06/). This should handle this case gracefully.

This also fixes detection of stage labels that include a space, such as devopsdata stores.

I also took the opportunity to refactor a bit the processor.

Expected impact & dry-runs

Closes #1309 (closed).

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review