matrix: ln -s retention m.room.retention

Ref: #15

matrix/m.room.power_levels/README.md

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+# Template power levels event for quick copy-pasting
+
+The main point I do this for is users `users`
+
+```json
+{
+    "users": {
+      "@mikaela:feneas.org": 100,
+      "@mikaela-matterbridge:feneas.org": 100,
+      "@Ciblia:matrix.org": 100,
+      "@mikaela:pirateriot.net": 100,
+      "@leon:the-apothecary.club": 100,
+      "@mikaela:liberta.casa": 100
+    }
+},
+```
+
+but here is `events` just in case. Please don't copy-paste directly, that may
+break things:
+
+```json
+{
+  "type": "m.room.power_levels",
+  "content": {
+    "invite": 100,
+    "events": {
+      "m.room.avatar": 100,
+      "im.vector.modular.widgets": 100,
+      "m.room.history_visibility": 100,
+      "m.room.name": 100,
+      "m.room.encryption": 100,
+      "m.room.tombstone": 100,
+      "m.room.canonical_alias": 100,
+      "m.room.power_levels": 100,
+      "m.room.topic": 100,
+      "m.room.retention": 100
+    },
+    "redact": 50,
+    "ban": 50,
+    "kick": 50,
+    "users_default": 0,
+    "events_default": 0,
+    "state_default": 50
+  },
+}
+```

matrix/m.room.retention

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+retention