Stripe promote on checkout

[ro-tex]

PULL REQUEST

Overview

Our typical payment flow starts with us creating a checkout session and redirecting the user to a Stripe-controlled page where they make the actual payment. After the payment is done, we wait for a push notification on our webhook endpoint. We process that notification and promote/demote the user accordingly.
This works well and is reliable because Stripe will keep sending the webhook event until they get a successful response.
The problem with it is that it might take a while and the user will be left without any notification what is going on.

This PR solves that problem by introducing a new endpoint which we can proactively call in order to check the status of a given checkout session and promote the user, if required. Note that we won't demote the user by calling this endpoint - that will be handled by the webhook endpoint. This new endpoint will only be used in order to speed up the user promotion process, so we get happy users.

Example for Visual Changes

Response JSON example:

{
    "created": 1658417273,
    "currentPeriodStart": 1658417273,
    "discount": {
        "amountOff": 0,
        "currency": "",
        "duration": "repeating",
        "durationInMonths": 3,
        "name": "test50%",
        "percentOff": 50
    },
    "id": "sub_1LO1I5IzjULiPWN6zatWjjxl",
    "plan": {
        "amount": 2000,
        "currency": "usd",
        "interval": "month",
        "intervalCount": 1,
        "product": {
            "description": "Pin up to 4TB",
            "name": "Skynet Pro"
        }
    },
    "startDate": 1658417273,
    "status": "active"
}

Checklist

• All git commits are signed. (REQUIRED)
• All new methods or updated methods have clear docstrings.
• Testing added or updated for new methods.
• Verified if any changes impact the WebPortal Health Checks.
• Appropriate documentation updated.
• Changelog file created.

Issues Closed

Closes SKY-1056

[linear]

SKY-1056 Implement an API endpoint to query the status of a payment

This endpoint should synchronously check the payment status with Stripe and update the user record if it was successful (so that we don't need to wait for the webhook call to arrive).

[peterjan]

looks good, would clean it up a bit but definitely close to done