Add `/server_promote` command

[bradleysigma]

This command should allow elevated users on the main server to promote themselves to an equivalent rank on the overflow servers.

• Admin, Developer and Bothelper can all promote themselves to Admin on the Emoji Farm severs;
• Admin can promote themselves to Admin, Developer and Bothelper on any of the regular servers;
• Developer can promote themselves to Developer and Bothelper on any of the regular servers;
• Bothelper can promote themselves to Bothelper on any of the regular servers;
• Everyone can also demote themselves from a rank they could promote themselves to.

This command only works in the #development channel, as a form of moderation.

[bradleysigma]

Also, this is untested, given that it needs the multi-server architecture to be testable.