templates: generalise auth templates for web and OIDC

Extract shared HTML/CSS design into a common template and create
generalised auth success and web auth templates that work for both
node registration and SSH check authentication flows.

Updates #1850

hscontrol/templates/auth_web.go

@@ -0,0 +1,21 @@
+package templates
+
+import (
+	"github.com/chasefleming/elem-go"
+)
+
+// AuthWeb renders a page that instructs an administrator to run a CLI command
+// to complete an authentication or registration flow.
+// It is used by both the registration and auth-approve web handlers.
+func AuthWeb(title, description, command string) *elem.Element {
+	return HtmlStructure(
+		elem.Title(nil, elem.Text(title+" - Headscale")),
+		mdTypesetBody(
+			headscaleLogo(),
+			H1(elem.Text(title)),
+			P(elem.Text(description)),
+			Pre(PreCode(command)),
+			pageFooter(),
+		),
+	)
+}