Add mithril to third party licenses

LICENSE-3RD-PARTY.md

@@ -16,3 +16,10 @@
     -   Based on the Font Awesome Icon `layer-group-solid` by Font Awesome, with modifications: Colors added
     -   Link: https://fontawesome.com/icons/layer-group?f=classic&s=solid
     -   License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)
+
+## Mithril.js
+
+-   Files: `portal/static/lib/mithril/*`
+    -   Author: Leo Horie
+    -   URL: https://github.com/MithrilJS/mithril.js
+    -   License: [MIT License](portal/static/lib/mithril/LICENSE)