[feat] Support for single room config

[TsFreddie]

Currently everything HAS to be configured in different room files, even if you only intend to use it for one room. And gamemode's vote are designed to have a prefix that may confuse any hosters.

Here's the idea that might help with hosters with teeworlds experience and intend to only host one room/one gamemode in their server:

• Allow gm_ prefix for gamemode config that get automatically applied to room config called 'default'. (e.g. gm_scorelimit 100
• If sv_room is enabled, /create or room votes allows creating room with the same config as the default.
• And maybe we should just remove vote prefix (maybe use a hashmap) altogether since the prefix don't provide anything useful for the users, and basically every votes will be room votes instead of global since most features have already been moved into instances.