betterbird: lock the ask to be the default mail client off, so it never shows at a start (hq issue 345)

Betterbird's own default asks at every start with a window to become the default for mail, news, feeds and
calendar files. The policy locks mail.shell.checkDefaultClient off, which wins over a profile's own value, and
betterbird_check says when a node would still ask.

modules/betterbird/install/distribution/policies.json

@@ -7,6 +7,12 @@
         "install_url": "file:///opt/betterbird/owl/owl.xpi",
         "updates_disabled": true
       }
+    },
+    "Preferences": {
+      "mail.shell.checkDefaultClient": {
+        "Value": false,
+        "Status": "locked"
+      }
     }
   }
 }