Improvements to sponsor map constraints

[applenick]

This PR addresses several long-standing issues related to sponsoring constraints and implements the following changes:

• Map variants are now considered under cooldown if any variant of said map has recently been played.
• Removed the 30-second match start requirement; players can now sponsor a map at any given point.
• Removed sponsor logic that discarded existing sponsor requests not within online player bounds. Now, all sponsor requests remain in the queue until they're able to play (if possible). Additionally, an alert is sent upon the selection of the sponsor map each map vote that informs the user and offers a quick button to cancel and select another map.
• Added an upper and lower limit offset to apply to the min/max bounds of the available maps only after the current match has ended.
• Improved visibility of the current online player range in error messages and the sponsor map list command.