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.

ingest/internal/config/config_test.go

@@ -10,8 +10,8 @@ func TestLoadDefaults(t *testing.T) {
 	if cfg.GRPC.ListenAddr != ":4317" {
 		t.Errorf("GRPC.ListenAddr = %q, want :4317", cfg.GRPC.ListenAddr)
 	}
-	if cfg.Redpanda.Topic != "sentry.logs.raw" {
-		t.Errorf("Redpanda.Topic = %q, want sentry.logs.raw", cfg.Redpanda.Topic)
+	if cfg.Redpanda.Topic != "cairnobs.logs.raw" {
+		t.Errorf("Redpanda.Topic = %q, want cairnobs.logs.raw", cfg.Redpanda.Topic)
 	}
 	if cfg.Batch.MaxSize != 500 {
 		t.Errorf("Batch.MaxSize = %d, want 500", cfg.Batch.MaxSize)