Add tree-kill dependency

Used for killing the running gotify instance.

ui/package-lock.json

@@ -11072,6 +11072,12 @@
         "punycode": "^2.1.0"
       }
     },
+    "tree-kill": {
+      "version": "1.2.0",
+      "resolved": "https://registry.npmjs.org/tree-kill/-/tree-kill-1.2.0.tgz",
+      "integrity": "sha512-DlX6dR0lOIRDFxI0mjL9IYg6OTncLm/Zt+JiBhE5OlFcAR8yc9S7FFXU9so0oda47frdM/JFsk7UjNt9vscKcg==",
+      "dev": true
+    },
     "trim-newlines": {
       "version": "1.0.0",
       "resolved": "https://registry.npmjs.org/trim-newlines/-/trim-newlines-1.0.0.tgz",

ui/package.json

@@ -47,6 +47,7 @@
     "prettier": "^1.14.2",
     "puppeteer": "^1.8.0",
     "rimraf": "^2.6.2",
+    "tree-kill": "^1.2.0",
     "tslint-sonarts": "^1.7.0",
     "typescript": "^3.0.1"
   }