Add opencollective payment POST /pay endpoint

[rakici]

This pull request implements the POST /pay endpoint as part of #366

Why ?
We want to allow jobsika users to create new job offers only once they have paid for the service.

Steps to verify:

• move to the backend folder with cd ./backend
• you need to get an api-key from open-collective, find out more
• cp .opencollective-env-example .opencollective-env
• add you api key to the .opencollective-env
• run make start-api to start the api
• then run the command curl -vsS -X POST -H 'Content-Type: application/json' -d '{"email":"[email protected]", "tier": "je suis con", "job_offer_id": "1"}' http://localhost:7000/pay | jq you should see something like this

{
  "tier_url": "https://opencollective.com/osscameroon/contribute/jobsika-joboffer-56908"
}

The tier_url is the link to the newly created payment tier.

[rakici]

We can now create a tier using the open collective API and return the URL of that tier to the client.

Next, we want to:

• Store the job_offer id associated with that payment request. We will use that ID to create the URL the customer will use to update their job_offer. (Add a joboffer_id field in the POST /pay Json body.
• Setup a webhook that will be notified once the payment is made and send an email to the customer with a link to update
• Delete the tier once the payment is done
• Write some tests
• Write a doc in a new doc folder, on how to run and test the payment system