fix: backend/package.json & backend/.snyk to reduce vulnerabilities

The following vulnerabilities are fixed with a Snyk patch:
- https://snyk.io/vuln/SNYK-JS-HTTPSPROXYAGENT-469131

backend/.snyk

@@ -0,0 +1,10 @@
+# Snyk (https://snyk.io) policy file, patches or ignores known vulnerabilities.
+version: v1.13.5
+ignore: {}
+# patches apply the minimum changes required to fix a vulnerability
+patch:
+  SNYK-JS-HTTPSPROXYAGENT-469131:
+    - mailgun-js > proxy-agent > https-proxy-agent:
+        patched: '2019-10-03T21:28:13.725Z'
+    - mailgun-js > proxy-agent > pac-proxy-agent > https-proxy-agent:
+        patched: '2019-10-03T21:28:13.725Z'

backend/package.json

@@ -10,7 +10,9 @@
   "scripts": {
     "dev": "nodemon",
     "docker:dev": "nodemon -L /opt/app",
-    "docker:prod": "node /opt/app"
+    "docker:prod": "node /opt/app",
+    "snyk-protect": "snyk protect",
+    "prepublish": "npm run snyk-protect"
   },
   "dependencies": {
     "async": "3.1.0",
@@ -30,6 +32,8 @@
     "request": "^2.88.0",
     "sha256": "^0.2.0",
     "socket.io": "^2.2.0",
-    "underscore": "^1.9.1"
-  }
+    "underscore": "^1.9.1",
+    "snyk": "^1.230.5"
+  },
+  "snyk": true
 }