Add setting to allow hiding all country flags

[peppy]

There have been enough requests for this at this point to implement it.

Required for some countries which have outlawed displaying some flags.

[bdach]

I guess the implementation works, but there is a weird quirk in UpdateableFlag that isn't making much sense to me:

osu/osu.Game/Users/Drawables/UpdateableFlag.cs

Lines 22 to 25 in 5285b35

	/// <summary>
	/// Whether to show a place holder on unknown country.
	/// </summary>
	public bool ShowPlaceholderOnUnknown = true;

Like... why is this configurable? It leads to this silliness:

	flags off	flags on
local leaderboard
user profile

wherein you get "?" in some places and just blank space in others...?

Like I don't get it why "?" is fine in some of these usages and some not and it creates this weird UI inconsistency. I'd probably just nuke that property?

[peppy]

Yeah I did wonder about that. I think always showing the "?" makes more sense right? Rather than an empty space.