Add mesh-mail-notifier, the default holder of mail-notifier (issue 800 part B)

ADR 0378 §9 as amended by ADR 0379: a module of its own, built from mesh-mail's
event shape, runs as its own account to hear mesh-mail's restricted MAIL stream
(ADR 0376 §2), shows each new inbox mail through node-notifier.send at most
once, folds more than three from one account into one notice, and emits its
own shown event.

modules/mesh-mail-notifier/module.json

@@ -0,0 +1,79 @@
+{
+  "module": "mesh-mail-notifier",
+  "version": "1",
+  "runs-as": "mesh-mail-notifier",
+  "claims": [
+    {
+      "name": "mail-notifier",
+      "scope": "node",
+      "serves": ["status"]
+    }
+  ],
+  "consumes": [
+    "mesh-mail.mail.*.message.added"
+  ],
+  "invokes": [
+    "seat:node-notifier.send",
+    "seat:mail-store.accounts"
+  ],
+  "emits": [
+    "shown"
+  ],
+  "state": [
+    {
+      "name": "shown",
+      "ttl-seconds": 2592000
+    }
+  ],
+  "tools": [
+    "mail-notifier.status"
+  ],
+  "own-secrets": {
+    "broker": "${dir:state}/broker"
+  },
+  "secrets-owner": "mesh-mail-notifier",
+  "resources": [
+    {
+      "id": "account",
+      "type": "user",
+      "name": "mesh-mail-notifier",
+      "shell": "/usr/bin/nologin",
+      "home": "/var/lib/mesh-mail-notifier"
+    },
+    {
+      "id": "state",
+      "type": "directory",
+      "mode": "0700",
+      "owner": "mesh-mail-notifier",
+      "place": "."
+    },
+    {
+      "id": "notify",
+      "type": "process",
+      "name": "mesh-mail-notifier-notify",
+      "artifact": "tools",
+      "run": [
+        "./mesh-mail-notifier",
+        "notify"
+      ],
+      "health": {
+        "kind": "unit"
+      }
+    }
+  ],
+  "build": {
+    "artifacts": [
+      {
+        "name": "tools",
+        "kind": "bundle",
+        "language": "go",
+        "system": "arch",
+        "from": "cmd/mesh-mail-notifier",
+        "binary": "mesh-mail-notifier",
+        "loads": [
+          "mesh-mail-notifier"
+        ]
+      }
+    ]
+  }
+}