Free up Usernames after deny/decline

What does this PR do?

Adds a new mod log type for decline, new Endpoint for declining which frees up the username from the used username table followed by it deleting the user.

This is essentially just a copied approve-user endpoint with the email text changed out and instead of marking the user approved it deletes the user and also frees up the username

Closes #752 (closed), #759 (closed)

Contribution Guidelines By submitting this merge request, you agree to follow our Contribution Guidelines

• I agree to follow this project's Contribution Guidelines
• I have made sure to test this pull request