add global settings page

[bnguyenngoc]

for admins only
in here, admin can:

• edit user (privilege for example)
• Allow challenge retry
• Allow challenge re submission for overkill
• Enable user registration
• Enable team registration
• Enable team joining
• Modify visibility of stunts for guests (not logged in)
• Modify visibility of teams for guests
• Modify visibility of Summary
• Modify message in Jumbotron