Implement Storing Tokens and Notifications

[joycrwu]

Overview

Implemented storing device tokens on the backend and sending notifications using the Expo API.

Changes Made

• Changed the UserModel to store device tokens in a string list
• Changed the SessionModel to store the device token associated with every session
• Changed the login endpoint to store the device token passed in by the frontend in the field of the user logging in
• Changed the logout endpoint to remove that particular device token from the user
• Modified the send notification endpoint such that it associates who to send notifications to with the input of an email
• Made appropriate database migrations

Test Coverage

• Manual testing on Postman. Received frontend device tokens from Jessie.
• First tested logging in
• Then getUserBySession should return the User with the appropriate new device token added in
• Hitting the send notification endpoint sent a push notification to my device and Jessie's device
• Hitting the logout endpoint
• Using another session that the user is active in, retrieving the user should find one less device token

[joycrwu]

i think we need to store device tokens in sessions so that when a user logs out/the session expires, we can get rid of that deviceToken and not send notifications to it anymore should the user be also logged in elsewhere with other sessions active; i store it here so that i can identify which device token to remove on the user when this session expires

[snajima]

let's get rid of list of deviceTokens in user, and add semicolons at the end of each line