Change Max Submissions to Unpenalized Submissions in Autolab API

[oliverli]

Description

Add max_unpenalized_submissions field to API's /courses/{course_name}/assessments/{assessment_name} endpoint

Motivation and Context

We want to show clients (especially in the CLI) the number of submissions they can make before they start incurring version penalties. Closes #1184

How Has This Been Tested?

Tested locally using Postman, with no version penalties, and with version penalties after 3 submissions

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation, which is located at Autolab Docs
• I have updated the documentation accordingly, included in this PR

Other issues / help required

If unsure, feel free to submit first and we'll help you along.

[victorhuangwq]

Tested the API. Correct result is returned after setting is updated as well.
LGTM!