[FEATURE] Added description field to spoiler and made the url field required

[janssensjelle]

Types of changes

What types of changes does your code introduce?
Put an x in the boxes that apply.

• Bugfix (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 not to work as expected).
• Documentation Update (if none of the other choices applies).

Proposed changes

Add a description field to the /spoiler command and made the URL field required as per request from @0xEmma . Removed the test for having no url as this is now required/handled by Discord.

Checklist

Put an x in the boxes that apply.

• I have read and followed the CONTRIBUTING.md
  doc.
• Lint and unit tests pass locally with my changes.
• I have added the necessary documentation (if appropriate).

Additional context

[janssensjelle]

This can cause a merge conflict due to a test being removed if this PR is merged after #133

[janssensjelle]

Closing this one as this change is now committed in PR #133