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.

enterprise/internal/saml/saml_test.go

@@ -27,7 +27,7 @@ func TestNewRejectsMissingConfig(t *testing.T) {
 }
 
 func TestNewRejectsMissingIDPMetadata(t *testing.T) {
-	_, err := New(Config{EntityID: "https://sentry.example.com/saml/metadata", ACSURL: "https://sentry.example.com/saml/acs"})
+	_, err := New(Config{EntityID: "https://cairnobs.example.com/saml/metadata", ACSURL: "https://cairnobs.example.com/saml/acs"})
 	if err == nil {
 		t.Fatalf("expected an error when IDPMetadata is missing")
 	}
@@ -40,8 +40,8 @@ func TestNewRejectsMissingIDPMetadata(t *testing.T) {
 // redirect rather than just "the code compiles."
 func TestLoginURLBuildsAgainstRealIDPMetadata(t *testing.T) {
 	sp, err := New(Config{
-		EntityID:    "https://sentry.example.com/saml/metadata",
-		ACSURL:      "https://sentry.example.com/saml/acs",
+		EntityID:    "https://cairnobs.example.com/saml/metadata",
+		ACSURL:      "https://cairnobs.example.com/saml/acs",
 		IDPMetadata: fakeIDPMetadata(),
 	})
 	if err != nil {