feat(mergify): Add mergify config (#234)

Also add venv to .gitignore so we don't accidentally check in our
python virtual environments.

.gitignore

@@ -15,3 +15,4 @@ var/
 .idea
 spinnaker-monitoring.iml
 generated_dashboards
+venv/

.mergify.yml

@@ -0,0 +1,39 @@
+pull_request_rules:
+  - name: Automatically merge on CI success and review
+    conditions:
+      - base=master
+      - status-success=build
+      - "label=ready to merge"
+      - "approved-reviews-by=@oss-approvers"
+    actions:
+      merge:
+        method: squash
+        strict: smart
+      label:
+        add: ["auto merged"]
+  - name: Automatically merge release branch changes on CI success and release manager review
+    conditions:
+      - base~=^release-
+      - status-success=build
+      - "label=ready to merge"
+      - "approved-reviews-by=@release-managers"
+    actions:
+      merge:
+        method: squash
+        strict: smart
+      label:
+        add: ["auto merged"]
+  # This rule exists to handle release branches that are still building using Travis CI instead of
+  # using Github actions. It can be deleted once all active release branches are running Github actions.
+  - name: Automatically merge release branch changes on Travis CI success and release manager review
+    conditions:
+      - base~=^release-
+      - status-success=continuous-integration/travis-ci/pr
+      - "label=ready to merge"
+      - "approved-reviews-by=@release-managers"
+    actions:
+      merge:
+        method: squash
+        strict: smart
+      label:
+        add: ["auto merged"]