feat: Add presence heartbeat for Matrix online status

- matrix-gateway: POST /internal/matrix/presence/online endpoint
- usePresenceHeartbeat hook with activity tracking
- Auto away after 5 min inactivity
- Offline on page close/visibility change
- Integrated in MatrixChatRoom component

node_modules/fast-json-stable-stringify/.travis.yml

@@ -0,0 +1,8 @@
+language: node_js
+node_js:
+  - "8"
+  - "10"
+  - "12"
+  - "13"
+after_script:
+  - coveralls < coverage/lcov.info