chore: Enable mergify for auto merging of PRs (#7810)

This PR enables mergify.io integration for automatic merging of Pull Requests.

Pull Requests will be merged if the following checks are passing

- Codacy
- Travis
- Semantic Title
- Snyk (package.json and requirements.txt)
- don't-merge label is not added
- PR is approved by atleast one person

.mergify.yml

@@ -0,0 +1,13 @@
+pull_request_rules:
+  - name: Automatic merge on CI success and review
+    conditions:
+      - status-success=Codacy/PR Quality Review
+      - status-success=Semantic Pull Request
+      - status-success=continuous-integration/travis-ci/pr
+      - status-success=security/snyk - package.json (frappe)
+      - status-success=security/snyk - requirements.txt (frappe)
+      - label!=don't-merge
+      - "#approved-reviews-by>=1"
+    actions:
+      merge:
+        method: merge