feat(qsl): credit line at the foot of the default QSL e-mail

"Designed & sent by OpsLog — https://github.com/GregTroar/OpsLog/releases/latest"
now closes the default body, so a fresh install sends it without being asked.

Put in the TEMPLATE, not appended at send time. Two consequences, both of them
the point: an operator who does not want it deletes the line once and it is
gone, and one who has already written their own body never sees it appear —
a stored template is returned verbatim and the default is only the fallback.
Appending it at send would have made it unremovable, which is not a thing to do
to somebody's outgoing mail.

The link is the human release page. updateCheckURL sits next to it in update.go
and answers JSON, which is the easy mistake here — a correspondent who clicks
the credit gets a download page, and a test says so.

qslcredit_test.go

@@ -0,0 +1,39 @@
+package main
+
+import (
+	"strings"
+	"testing"
+)
+
+// The default QSL e-mail closes with a credit line carrying the download page.
+//
+// It is part of the BODY TEMPLATE, deliberately: an operator who does not want
+// it deletes it once, and one who already wrote their own body never sees it,
+// because a stored template is returned verbatim and the default is only the
+// fallback. Appending it at send time would have made it unremovable.
+func TestQSLCreditIsInTheDefaultBody(t *testing.T) {
+	if !strings.HasSuffix(defaultQSLEmailBody, qslCredit) {
+		t.Errorf("the default QSL e-mail does not end with the credit line:\n%q", defaultQSLEmailBody)
+	}
+	// The body still has to be a usable template — the credit is added to it,
+	// not in place of it.
+	for _, v := range []string{"{DATE}", "{BAND}", "{MODE}", "{MYCALL}"} {
+		if !strings.Contains(defaultQSLEmailBody, v) {
+			t.Errorf("the default QSL e-mail lost %s", v)
+		}
+	}
+}
+
+// The link must be the page a person can read. updateCheckURL sits beside it and
+// answers JSON — sending a correspondent there is the easy mistake to make.
+func TestQSLCreditLinksTheHumanPage(t *testing.T) {
+	if !strings.Contains(qslCredit, releasesPageURL) {
+		t.Errorf("credit %q does not carry the releases page", qslCredit)
+	}
+	if strings.Contains(qslCredit, "api.github.com") {
+		t.Errorf("credit %q points at the update API, which answers JSON to whoever clicks it", qslCredit)
+	}
+	if !strings.Contains(qslCredit, "OpsLog") {
+		t.Errorf("credit %q does not name OpsLog", qslCredit)
+	}
+}