Allow UI modding for HUD only with Lua disabled

[slipher]

@DolceTriade suggested in a comment an idea for how to enable limited UI modding: disable Lua in the HUD context and allow modding of the HUD only. Without Lua it would be a lot harder to abuse UI modding for cheating as there should be no way to send commands, get access to information (e.g. locations of non-beaconed entities) not intended for the UI, or generally break out of the sandbox.

I haven't verified that RmlUi allows enabling Lua on a per-context basis.

I see one serious drawback to this idea, which is that it would prevent us from using Lua in the "official" HUD in the future. Some HUD elements have drawing routines implemented in C++ which send drawing commands directly to the engine, bypassing RmlUi. If we were implementing those elements today it's likely we'd use RmlUi+Lua.