Implement POST /contract route (artists only)

[gcarvellas]

Depends on #2 (currently in a pull request)
Part of #4

About this issue:

We are creating a POST route called /contract, where this route will accept a request body, parse the body, validate the body, and send it to the docusign contract generator.

CODE STUB:

In the branch 11-implement-createcontract-route-for-artists linked with this issue, there is a file /backend/create_contract.py. Write your code there.

How to Call Docusign Contract Generator:

In backend/docusign/test_contract.py, the function Docusign.create_contract returns a contract id if created successfully. Otherwise, returns an exception

Request Fields:

• contractType: String (For this issue, the only accepted value should be "artist")
• month: String
• helperBadgeQt: Integer (Number of helper badges needed)
• additionalChairsQt: Integer (Number of additional chairs needed)
• artistNumber: Integer (artist's phone number)
• helper1Number: Integer (helper's phone number)
• shortenedYear: Integer (shortened format of the year. Ex: if the year is 2022, this value will be 22.)
• day: Integer (day of the month)
• signerName: String (Name of the signer)
• signerEmail: String (Email of the signer)
• approverName: String (Name of the approver)
• approverEmail: String (Email of the approver)

Response Field:

Status Code 200

{
     contractId: {ENTER CONTRACT ID HERE}
}

Status Code 500

{
     error: {ENTER EXCEPTION MESSAGE OR CUSTOM ERROR MESSAGE}
}

Status Code 400

{
     error: {ENTER EXCEPTION MESSAGE OR CUSTOM ERROR MESSAGE}
}

[gcarvellas]

What's left in this issue:

1. Verify that all inputs are passed in properly
2. Verify that our inputs are aligned with the docusign envelope inputs
3. Test actually making a post route and verifying that it works with docusign.