Add mattermost authentication

package.json

@@ -76,6 +76,7 @@
     "markdown-pdf": "^7.0.0",
     "mathjax": "~2.7.0",
     "mermaid": "~7.1.0",
+    "mattermost": "^3.4.0",
     "meta-marked": "^0.4.2",
     "method-override": "^2.3.7",
     "moment": "^2.17.1",
@@ -91,6 +92,7 @@
     "passport-google-oauth20": "^1.0.0",
     "passport-ldapauth": "^0.6.0",
     "passport-local": "^1.0.0",
+    "passport-oauth2": "^1.4.0",
     "passport-twitter": "^1.0.4",
     "passport.socketio": "^3.7.0",
     "pdfobject": "^2.0.201604172",