chore: migrate mail themes to shared framework and locked upstream inputs

tools/upstream/issues.go

@@ -0,0 +1,8 @@
+package upstream
+
+// ReviewedFormattingIssue is deliberately matched by language, key and exact
+// upstream text. It cannot hide new defects after a snapshot update. Preview
+// preserves Gitea's output and advertises this upstream defect to the user.
+func ReviewedFormattingIssue(lang, key, value string) bool {
+	return lang == "pl-PL" && key == "mail.team_invite.text_1" && value == "%[1]zaprasza Cię do dołączenia do zespołu %[2]s w organizacji %[3]s."
+}