Add foreign key relationship between SiteModel and NotificationModel

[AmandaRiu]

Fixes #1284 by adding a foreign key for local_site_id that depends on SiteModel. If the matching site is deleted, the records matching that local_site_id in NotificationModel will deleted automatically.

This PR includes:

• Update to NotificationModelBuilder to add the foreign key
• Update to WellSqlConfig to drop NotificationModel on upgrade and recreate the table with the foreign key
• Updated tests to account for the new SiteModel db dependency as well as a test to verify that the on delete cascade works
• New DELETE FIRST SITE (DB ONLY) button in the Sites section of the example app. This allows for testing by:

1. Log in with a Woo account
2. In the Notifications page, fetch all notifications from the API
3. In the Sites page, delete the first site from the db
4. Verify the NotificationModel table using Stethos.

[anitaa1990]

Looks great!