feat(donate): updated donate page and plans

- [x] two column layout for the page.
- [x] amount to learning minutes mapping for contribution impact.
- [x] handle one-time and recurring stripe subscription charges.
- [x] server side validation of donate forms.
- [x] prevent multiple subscriptions and onetime donations per user.

api-server/server/models/donation.json

@@ -28,6 +28,9 @@
       "required": true,
       "description": "The donation amount in cents"
     },
+    "duration": {
+      "type": "string"
+    },
     "startDate": {
       "type": "DateString",
       "required": true