Support attached images when issuing warnings to user

[Sasiko]

The purpose is to allow moderators to add an attached image when issuing a warning to a user. Usecase would be a screenshot of the offense like DM advertisement

/warnings will show attached image as part of the warning "reason".

[ashishjh-bst]

First, you must read contributing guidelines, and know how to raise a PR.
Second, the attachment link would become invalid as soon as the original moderation message is deleted, so this just extends the functionality to store dud urls.

[ashishjh-bst]

Also, this doesn't compile, please build your code locally before sending in PRs.

# github.com/botlabs-gg/yagpdb/v2/moderation
../../moderation/commands.go:900:114: not enough arguments in call to WarnUser
        have (*Config, int64, *dstate.ChannelState, *discordgo.Message, *discordgo.User, *discordgo.User, string)
        want (*Config, int64, *dstate.ChannelState, *discordgo.Message, *discordgo.User, *discordgo.User, string, bool)