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

The following vulnerabilities are fixed with a Snyk patch:
- https://snyk.io/vuln/SNYK-JS-LODASH-567746

backend/.snyk

@@ -1,5 +1,5 @@
 # Snyk (https://snyk.io) policy file, patches or ignores known vulnerabilities.
-version: v1.13.5
+version: v1.14.1
 ignore: {}
 # patches apply the minimum changes required to fix a vulnerability
 patch:
@@ -8,3 +8,6 @@ patch:
         patched: '2019-10-03T21:28:13.725Z'
     - mailgun-js > proxy-agent > pac-proxy-agent > https-proxy-agent:
         patched: '2019-10-03T21:28:13.725Z'
+  SNYK-JS-LODASH-567746:
+    - mailgun-js > async > lodash:
+        patched: '2020-04-30T21:29:23.212Z'