Add stale probot (#1609)

.github/probots.yml

@@ -1,2 +1,3 @@
 enabled:
-  - cla
+- cla
+- probot-stale

.github/stale.yml

@@ -0,0 +1,34 @@
+# Number of days of inactivity before an Issue or Pull Request becomes stale.
+daysUntilStale: 90
+
+# Number of days of inactivity before an Issue or Pull Request with the stale label is closed.
+# Set to false to disable. If disabled, issues still need to be closed manually, but will remain marked as stale.
+daysUntilClose: 30
+
+# Issues or Pull Requests with these labels will never be considered stale. Set to `[]` to disable.
+exemptLabels: ["stale/exempt", "pinned"]
+
+# Label to use when marking as stale.
+staleLabel: stale
+
+# Comment to post when marking as stale. Set to `false` to disable.
+issues:
+  markComment: >
+    Thank you for taking the time to raise this issue. However, it has
+    not had any activity on it in the past 90 days and will be closed in
+    30 days if no updates occur.
+
+    Please check if the master branch has already resolved the issue
+    since it was raised. If you believe the issue is still valid and you
+    would like input from the maintainers then please comment to ask for
+    it to be reviewed.
+
+pulls:
+  markComment: >
+    Thank you for your contribution! However, this pull request has not
+    had any activity in the past 90 days and will be closed in 30 days if
+    no updates occur.
+
+    If you believe the changes are still valid then please verify your
+    branch has no conflicts with master and rebase if needed. If you
+    are awaiting a (re-)review then please let us know.