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.

changelog.json

@@ -4,11 +4,13 @@
     "date": "",
     "en": [
       "Right-click: update the US county of the selected contacts from the ULS database, replacing a county since renamed or abolished.",
-      "A QSO logged from WSJT-X, MSHV or a net now appears in Recent QSOs at once, instead of waiting for a delayed auto-upload to send it."
+      "A QSO logged from WSJT-X, MSHV or a net now appears in Recent QSOs at once, instead of waiting for a delayed auto-upload to send it.",
+      "New installs: the default QSL e-mail ends with a credit line and a link to OpsLog. It is part of the template, so delete it if you'd rather not."
     ],
     "fr": [
       "Clic droit : mettre à jour le comté US des contacts sélectionnés depuis la base ULS, pour remplacer un comté renommé ou supprimé.",
-      "Un QSO logué depuis WSJT-X, MSHV ou un net apparaît aussitôt dans les QSO récents, sans attendre l’envoi d’un upload automatique différé."
+      "Un QSO logué depuis WSJT-X, MSHV ou un net apparaît aussitôt dans les QSO récents, sans attendre l’envoi d’un upload automatique différé.",
+      "Nouvelles installations : le mail QSL par défaut se termine par une ligne de crédit et un lien vers OpsLog. Elle fait partie du modèle, supprimable."
     ]
   },
   {