chore(mergify): automerge dependabot, allow provisional approval (aws…

…#4043)

Two changes to the Mergify config (hoping that I've done it correctly):

- Automatically merge commits from dependabot. Having to go through
  approve/rebase/approve cycles is tiresome.
- Don't dimiss stale approvals if the PR author is someone from the core
  team. This allows doing provisional approvals ("it's okay but change
  this one thing before you merge").

.mergify.yml

@@ -10,7 +10,6 @@ pull_request_rules:
         method: squash
         strict_method: merge
       delete_head_branch: {}
-      dismiss_reviews: {}
     conditions:
       - author!=dependabot[bot]
       - author!=dependabot-preview[bot]
@@ -23,6 +22,19 @@ pull_request_rules:
       - "#changes-requested-reviews-by=0"
       - status-success=AWS CodeBuild us-east-1 (aws-cdk-build)
       - status-success=Semantic Pull Request
+  - name: automatically merge dependabot
+    actions:
+      comment:
+        message: Thanks for keeping us safe dependabot! ❤️
+      merge:
+        strict: smart
+        method: squash
+        strict_method: merge
+      delete_head_branch: {}
+    conditions:
+      - author=dependabot[bot]
+      - author=dependabot-preview[bot]
+      - status-success=AWS CodeBuild us-east-1 (aws-cdk-build)
   - name: comment checklist
     actions:
       label:
@@ -69,6 +81,7 @@ pull_request_rules:
     conditions:
       - author!=dependabot[bot]
       - author!=dependabot-preview[bot]
+      - author!=@aws/aws-cdk-core
       - base=master
   - name: if fails conventional commits
     actions: