Add invite routes to backend and frontend; implement routing and UI components for invitation management

backend/src/models/index.js

@@ -15,6 +15,7 @@ const Sick = require('./Sick');
 const SickType = require('./SickType');
 const Timefix = require('./Timefix');
 const Timewish = require('./Timewish');
+const Invitation = require('./Invitation');
 
 module.exports = {
   User,
@@ -28,6 +29,7 @@ module.exports = {
   Sick,
   SickType,
   Timefix,
-  Timewish
+  Timewish,
+  Invitation
 };