[TF2] Let servers set mat_supportflashlight to 1 for clients

[KaelaSavia]

It would make a lot of horror gamemodes and maps doable in multiplayer.

There's already built-in support for servers setting client convars (such as host_timescale being set by Ninja Spy boss in FF2 for time slowdown). I don't believe there would be any big issues if it was extended to mat_supportflashlight