Remove everything specific to a Gitea deployment (closes #209)

Delete the Gitea compose setup, its document, its two scripts and their
make targets. Drop the Core Rule Set exclusions written for Gitea: the
ten query parameters skipped by rules 930120, 932160 and 932260, and the
gitea_flash and redirect_to cookies. The remaining changes are
renumbered, five in all. Documents and tests use neutral examples.

Model: opus-5-5

internal/config/config.go

@@ -409,7 +409,7 @@ var (
 	errNoCertificate = errors.New("holds no PEM certificate")
 	errNotFacility   = errors.New("is not a syslog facility such as local0 or daemon")
 	errNotAppName    = errors.New(
-		"is not 1 to 48 printable ASCII characters without a space, such as gitea")
+		"is not 1 to 48 printable ASCII characters without a space, such as blog")
 	errSetTwice      = errors.New("set only one of them")
 	errNotWebhookURL = errors.New(
 		"is not an http or https URL without a user or a fragment, " +