Finish the Cairn OBS rename through services, docs, and assets

The rename commit before this one covered module paths and the obvious
user-facing strings; this is the rest of it -- the places where "sentry"
was a default value, a filename, or a picture rather than a word in a
sentence.

Defaults that changed: CLICKHOUSE_DATABASE (sentry -> cairnobs),
POSTGRES_DATABASE (sentry_metadata -> cairnobs_metadata), and
POSTGRES_USERNAME (sentry -> cairnobs), across api/alerting/ingest and
the enterprise binaries, plus the compose files and migrate scripts that
create those objects. These are *defaults*, so a deployment that sets
them explicitly is unaffected -- but any deployment relying on the old
defaults must have its environment updated before it picks this up, or
it will come up pointing at a database that doesn't exist.

Also: the light-mode logo variants (the dark ones existed alone, so the
landing page and sidebar rendered a dark mark on a light background),
regenerated favicons, and the docs/README/threat-model prose that still
said Sentry.

metadata/docker-compose.yml

@@ -9,8 +9,8 @@ services:
     ports:
       - "5432:5432"
     environment:
-      POSTGRES_DB: sentry_metadata
-      POSTGRES_USER: sentry
+      POSTGRES_DB: cairnobs_metadata
+      POSTGRES_USER: cairnobs
       POSTGRES_PASSWORD: "cairnobs-dev-only" # not a real secret, see root docker-compose.yml
     volumes:
       - metadata-postgres-data:/var/lib/postgresql/data