Use UTF-8 instead of narrow

Use wide_to_utf8 and utf8_to_wide instead of wide_to_narrow and narrow_to_wide at almost all places.
Only exceptions: test functions for narrow conversion, and chat, which is done in a separate commit.

src/server.cpp

@@ -1183,7 +1183,7 @@ void Server::ProcessData(NetworkPacket *pkt)
 					<< ban_name << std::endl;
 			// This actually doesn't seem to transfer to the client
 			DenyAccess_Legacy(peer_id, L"Your ip is banned. Banned name was "
-					+ narrow_to_wide(ban_name));
+					+ utf8_to_wide(ban_name));
 			return;
 		}
 	}