Channel based bans

[S-S-X]

Implements #29

Allows banned players to read channel but disallows sending messages.

Might work, didn't test, didn't try if minetest can load this. So might work but I've no idea really...

Requires enabling extension with following configuration (minetest conf file):

beerchat.enable_ban = true

Commands:

Command	What it does
/channel_ban player_name [player_name ...]	Add bans for players on current channel
/channel_unban [player_name ...]	Removes bans for players on current channel
/channel_unban	Lists banned names on current channel

Missing stuff:

• Should allow channel name with /channel_ban #main Sam, currently always operates on active channel.
• Same for /channel_unban #main Sam
• Maybe add optional flag to also ban reading channel instead of just disallowing writing to channel.
  Reading is allowed simply to allow reading public info channels and currently this is most important actual use case for channel bans.
  In other words to prevent writing to some public channels like main chat channel without actually removing player from that channel.

Does not check if player exists, this should allow also banning external names. Not sure if this is really needed because there's also /remote_mute from another extension (bit different but still similar).

[S-S-X]

Plan was to extend this later with option to also ban reading channel / joining channel. Possibly also include /channel_kick here.

But not here, not going to add any of those additional features until fate of this PR is decided. This is what is needed to solve real issues, imaginary issues can be solved later.