Fix Sparta Sync command and allow for local testing

[Fenntasy]

Description

This PR has two main goals:

• fixing the fact that sparta sync overrides the repository even when the exercises already exists with the same SHA,
• and allowing the use of a local instance of Sparta API.

sparta init has now two hidden flags: --force (or -f) and --spartaURL="url".
Both have default values so it will not change the usage for students.
--spartaURL is there to change the URL that the CLI will ask for the calendar (which can now be set to our .local instance) and will record that in the config.json that the CLI generates at init
--force is a convenience option to recreate an exercises directory when calling sparta init.

Motivation and Context

We couldn't tell the previous students to use sparta sync even if they missed some day because it would have overwritten all their exercises.

How Has This Been Tested?

Manually locally

Types of changes

• Chore (non-breaking change which refactors / improves the existing code base)
• 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 code follows the code style of this project.
• ✅ : My change requires a change to the documentation.
• ✅ : I have updated the documentation accordingly.
• ✅ : I have read the CONTRIBUTING document.
• 🔴 : I have added tests to cover my changes.
• ✅ : All new and existing tests passed.