Add command to ping staff

[emmiegit]

Adds a command that alerts staff in the bot as opposed to using role mentions. Command !staff ping, (to allow for other group commands like !staff list).

• Allows easier pinging of staff when they are split into multiple roles
• Allows you to disable role pings, which have no restriction on their use
• Ratelimiting. Can be configurable per guild, with a default set in the config file. Gives minutes until ping can be used again.
• Requires a reason before it will ping
• Denies reasons less than 3 characters long
• Can add users to a ping blacklist
• Won't work if the user has the guest, mute, or jail roles
• Sends a message that pings individual staff rather than use a role.
• Only pings online staff (level can be set by guild: available, available or away, not offline, any)
• Edits (or delete-replaces) the mentions and writes the user who requested it and their message