Fix notifications access from service

[lorenyu]

Ticket

Contributes to #792

Changes

Fix ability for ECS service to access Pinpoint:

• Add VPC endpoints for Pinpoint services
• Add IAM policy for accessing Pinpoint and SES to service

Also add notifications test endpoint to example app for testing:

• Add /notifications page where you can enter an email address and get a test notification to that email address
• Add links to the homepage to all the test pages in the example app

Context for reviewers

I added /notifications endpoint since it will be useful to test changes to notifications infrastructure, but while testing I discovered some access bugs

Testing

see navapbc/platform-test#154