Store reason for deleting a sound

[ffont]

We should add a deletion_reason field to the DeletedSound object to store the reason why a sound is deleted. This could be one of:

• Spam
• Offensive
• Illegal (as reported by the community)
• Illegal (spotted by a moderator)
• Deleted by the author

Whenever possible, we should fill in this field automatically. In the admin we should create delete buttons for each of the reasons (or find an alternative way to pass the reason to the deletion function).