maintainers: add known alumni

config/maintainers.json

 {
   "maintainers": [
     {
+      "github_username": "jtigger",
+      "show_on_website": false,
+      "alumnus": true,
+      "name": null,
+      "bio": null,
+      "link_text": null,
+      "link_url": null,
+      "avatar_url": null
+    },
+    {
+      "github_username": "matthewmorgan",
+      "show_on_website": false,
+      "alumnus": true,
+      "name": null,
+      "bio": null,
+      "link_text": null,
+      "link_url": null,
+      "avatar_url": null
+    },
+    {
       "github_username": "sit",
       "show_on_website": false,
       "alumnus": false,