Use Lock to automatically lock old closed issues

See https://www.henryzoo.com/blog/2018/05/06/an-issue-with-issues.html
for rationale.

Author: Michael Jackson

.github/lock.yml

@@ -0,0 +1,25 @@
+# Configuration for lock-threads - https://github.com/dessant/lock-threads
+
+# Number of days of inactivity before a closed issue or pull request is locked
+daysUntilLock: 60
+
+# Issues and pull requests with these labels will not be locked. Set to `[]` to disable
+exemptLabels: []
+
+# Label to add before locking, such as `outdated`. Set to `false` to disable
+lockLabel: false
+
+# Comment to post before locking. Set to `false` to disable
+lockComment: false
+
+# Limit to only `issues` or `pulls`
+# only: issues
+
+# Optionally, specify configuration settings just for `issues` or `pulls`
+# issues:
+#   exemptLabels:
+#     - help-wanted
+#   lockLabel: outdated
+
+# pulls:
+#   daysUntilLock: 30