Add billboard at base with server logo and rules

[jlillis]

There should be a large billboard at base that can be used to display the server's name, logo, and rules.

• Add billboard object (d_blufor_base_billboard) to base(s)

• • Takistan
• • Sahrani
• • Altis
• • Stratis

• Add billboard image file to repo and texture replacement code to init.sqf
  For upstream PR: use existing domination logo by default
  For TF Spartan: use community-created image