Single attachment enforcement

[e-a-h]

For channels like screenshot challenge entry, a feature that will allow only one attachment per member in a given channel.

• channel configuration to add/remove channel
• store user id of entrants, message id, and attachment url
• when member adds another attachment, prompt: "You have already submitted. Do you want to replace your older submission? [image]" [yes] [no]
• single message with more than one attachment to be rejected/deleted/warned