move controller tweaks to server

(fixup of 736ceaa6c7e81882c9c5b2e932307b1f1ecb3bfd)

following discussion with Solition, we move all "controller tweaks"
i.e. assignments of networked side to human on the matching client
at start of game, to be server decisions, performed concurrent with
start of game.

in fact the controller tweaks are performed using
game::change_controller, which is modified so that it doesn't try to
make server messages before the game has started.

when the server receives [start_game] from the host, the server will
change_controller for every side, sending corresponding messages to
all players and observers, which will update them as appropriate.
it will also rewrite level_ so that human sides are "network" and
ai sides are "network_ai", making the level_ correct for an observer
who subsequently joins.

we remove the unneeded code in playcampaign.cpp
[replay_start] in server replays should match client replays

we delete [multiplayer] from the level in order to construct the
contents of [replay_start], exactly as is done in mp_game_utils.cpp
here:

https://github.com/wesnoth/wesnoth/blob/a4f47a63c7b2c55fd7d8d3cd425ede256b39acdd/src/mp_game_utils.cpp#L188
fixup response.clone()